All markets are prone to bubbles and the transfer market for footballers, where individuals are treated like commodities and valued by pundits and fans alike, is no different. This means that several players are significantly undervalued and, at the other end, overvalued.

Football Whispers' unique Player Value algorithm estimates a footballer's market value based on their current and future performance. To do so, we take into account market inflation, the player's age, the competitions in which they've played and the quality of their performances. Read this for more insight into our methodology.

Traditionally, "bargains" have been in the sub-£10 million category: Michu's £2m signing for Swansea is the archetypal value signing. However, there is also value to be had in the market for the world's top performing players. Equally, there are players at the top end of the market who shouldn't sell for what they do.

In this article, we are going to focus on three elite players at both ends of the spectrum.

The most undervalued

Christian Eriksen, Tottenham and Denmark

Eriksen isn't the most exciting player when it comes to highlights but his productivity makes him a steal. Duncan Boddy, Football Whispers

The £12m that Tottenham Hotspur paid Ajax in August 2013 for Eriksen ranks as one of Daniel Levy's savviest transfers during his time at Spurs' helm. The midfielder was a star in the Eredivisie, a league notorious for developing attacking talent, and yet there was little competition for the Denmark international's signature at the time.

Since that move Eriksen has blossomed into one of the best midfielders in the world, and is possibly the Premier League's most underrated player. While attacking talents like Alexis Sanchez, Kevin De Bruyne and Eden Hazard have received universal acclaim in their time in England, Eriksen's quiet consistency has gone under the radar -- until this season at least. In his 194 games for Spurs, he has scored 46 and assisted 60 goals. He has also developed a defensive work-rate that allows him to be pivotal in Mauricio Pochettino's intense counter-pressing system.

While many modern attackers may lament the increased defensive responsibility that is becoming the norm, Eriksen has embraced it and evolved into an exceptionally rounded player, one that is able to play across the front line as well central midfield.

Most importantly, the two-footed playmaker is only 25 years old, which means he's yet to enter his peak years. We currently value Eriksen at £66m, which is probably a low-ball estimate.

Mohamed Salah, Liverpool and Egypt

Liverpool paid handsomely for Roma but his sizable returns at the attacking end make him worth much, much more. Duncan Boddy, Football Whispers

Salah was bought by Liverpool from Roma for a fee in the region of £34m last summer, but let's pretend we don't know that given transfer fees can be influenced by random factors; they don't necessarily correlate with what a player's true value actually is.

The Egypt international moved to Roma permanently from Chelsea for just £13.5million in July 2016 after being branded a flop at Stamford Bridge. However, he had spent the previous 18 months on loan in Serie A with Fiorentina and i Giallorossi. In 83 games for Roma, Salah scored 34 goals and assisted a further 24. His astonishing return quickly gained him a reputation as one of the most dangerous players in Italian football.

His almost unrivalled ability as a goal-scoring inside forward during his two seasons in Rome would separate Salah from the majority of European wingers; while left footed wingers playing on the right, such as Arjen Robben, are a rare breed. Salah was a perfect fit for Liverpool's fast-breaking, counter-pressing system, and it's a surprise they managed to sign the 25-year-old for that price given he is entering the prime of his career.

In 17 matches for his new club in all competitions, Salah has scored 12 goals and assisted four. He is well on his way to being brandished the signing of the summer. We value him at £67m, with this estimate likely to rise significantly after this season is over.

Marek Hamsik, Napoli and Slovakia

Hamsik cost less than five million pounds but could sell for 10 times that if Napoli were interested. Duncan Boddy, Football Whispers

Napoli's decision to part with £4.9m to sign Marek Hamsik from Brescia in 2007 has proven to be one of the all-time great pieces of business in Serie A. The central midfielder, with his iconic mohawk, has become the team's central pillar ever since even while other stars like Edinson Cavani, Gonzalo Higuain and Ezequiel Lavezzi have come and gone for significant profits.

"Marekiaro" (as he is affectionately known in Naples) is just one goal away from Maradona's record of 115 goals in the famous blue shirt. In 2013, the Slovakia international claimed that he wanted to "become Napoli's Totti," which may explain the lack of transfer interest in him from clubs on the continent. Since then, though, the club has experienced a resurgence under Maurizio Sarri and Hamsik, along with some of his teammates, may want to capitalise on this level of performance with a move to one of the top Champions League clubs.

Regardless of the likelihood of a transfer, our performance-based valuation model ranks Hamsik as one of the best in his position, despite little to no recognition globally compared with his competition. His ability as an attacking central midfielder, along with his work-rate, make him uniquely suitable to the action-packed tactical systems that are increasingly popular in Europe's elite teams right now.

Aged 30, Hamsik is coming towards the tail-end of his prime but it says something about how well he still performs that our model values at him £55m, even after factoring in his age. Given Manchester City's desire to replace Yaya Toure, and Pep Guardiola's repeated praise for Sarri's high-octane team, Hamsik could be a perfect fit for them in the summer.

The most overvalued

Philippe Coutinho, Liverpool and Brazil

Coutinho's inflated value might be due to Barcelona's unique needs rather than anything else. Duncan Boddy, Football Whispers

Barcelona's reported bids in excess of £120m for Philippe Coutinho this summer illustrate the distinction between "value" and "cost" in the transfer market.

The context is important. In the deal of the summer, PSG activated Neymar's £200m release clause and lured him away from Barcelona; the icing on top of a very lucrative cake was that the Brazil international would finally be able to step out of Leo Messi's shadow. Embarrassed by the loss of one superstar, and loaded with newfound riches, Barcelona wanted to replace Neymar with another elite player and set their sights on Liverpool's Coutinho.

Since joining the Reds from Inter Milan for a bargain £8.5m in 2013, Coutinho has developed into one of the Premier League's best attacking players, which is being recognised in our estimation of his market value. At £75m, we have Coutinho as the 14th-most valuable player in the world, predominantly because he is performing at a level that is largely unrivalled.

Nonetheless, the fee Barcelona were willing to pay for Coutinho appears exorbitant for a player of his ability. Christian Eriksen, Thiago Alcantara, and Koke are similar in value, age, and ability, and may all have been available for less than £100m. But a steadfast desire to sign a player from potential buyers often clashes with an even more stubborn desire to keep said player by the selling club. It leads to fees quickly deviating from a player's true value due to the nature of negotiation, and that's what happened with Coutinho.

Ousmane Dembele, Borussia Dortmund and France

When Barcelona couldn't land Coutinho, they famously paid big -- too big -- for Dembele. Duncan Boddy, Football Whispers

In another of the summer's transfer sagas, Barcelona pushed hard to sign Borussia Dortmund winger Dembele and eventually got their man for an exorbitant £94.5m fee, 12 months earlier the France international joined the German side for just £13m.

Much like Coutinho, our Player Value model ranked Dembele as one of the world's most valuable footballers this summer after his excellent performances for Dortmund at the tender age of 20. The two-footed winger, whom our model valued at £60m, was prolific in his sole season at Signal Iduna Park, scoring 10 goals and assisting a further 20 in 50 appearances for the Black and Yellows. There was no doubting Dembele's talent and Barcelona's interest was hardly surprising, but their desperation to replace Neymar forced the Catalan club to drastically overpay.

Again, the fee agreed is largely down to nature of negotiation: Dortmund were entirely aware of Barcelona's position and ensured they were able to extract a fee unimaginable just a year ago, one that is well in excess of his true value.

Kylian Mbappe, Paris Saint-Germain and France

Mbappe is the most in-demand forward in world football right now and the price PSG paid reflects that. Duncan Boddy, Football Whispers

Kylian Mbappe set the footballing world alight with his explosive debut season for Monaco, performing both domestically, as the club won Ligue 1, and in the Champions League, where they reached the semi-final. Our "wonder-kid premium" rewards the performance of teenagers disproportionately, but the scarcity of 17-year-olds performing at the level that Mbappe did makes it hard for us to value him properly. Our Performance model rated him as Monaco's best player last season by some margin, and one of the best in Ligue 1.

Our model's estimate (£67m) is probably an under-valuation, though even our most optimistic valuation would not come close to the £166m deal that PSG negotiated to secure the Frenchman's services. The Parisian club's intention was never to get value for their money, although they may still do so if Mbappe becomes the world star many expect. For PSG, a club where money is seemingly no object, the Mbappe and Neymar transfers were about making a statement. They did exactly that.