Story highlights Frankel has won $1.8 million in prize money so far but he could be worth $155 million in stud

Top yearlings used to fetch $10 million at auction but the market has deflated in recent years

Black Caviar is the highest-rated filly of all time according to racehorse performance experts

Both horses are mini marketing enterprises with a range of branded clothing and gifts

Frankel and Black Caviar. The burning question in the horse racing world is whether its two biggest stars will ever get together.

While that once meant "Who would win if they went head to head?" now all the talk is about what they might produce given time and a bit of privacy.

And, just as importantly, how much would that coupling be worth in the mega-bucks breeding stakes?

Both horses have swept all before them in illustrious careers. Frankel has posted nearly $3 million in prize money for the Juddmonte Farm stables, with an unbeaten run in 12 encounters. Black Caviar has also done well for her owners, with 22 wins on courses in Australia and the UK in the past four years.

Frankel is now rated as the best performer on the flat in the world by race statistics company Timeform -- which since 1948 has been using a mathematical equation comparing race results to the weight of the horse plus distance run to create a hierarchy which is used in many form guides for racegoers and owners.

While performance on the turf is not always a leading indication of performance in retirement, top bloodstock experts say Frankel's lifetime value in stud could be around $155 million.

"Frankel will have the pick of the best mares when it is time for him to stud, and there is no mare out there at the moment attracting as much attention as Black Caviar," says Johnny McKeever, who has 25 years of experience in buying horses which have gone on to win major Group One meets around the world.

"If you own a good mare and you can afford the stud fees Frankel will command, why wouldn't you want to put forward your horse to breed with him?" he told CNN.

And that initial stud fee is set to break the current world record held by Irish horse Sea The Stars, who earned nearly $7 million in prize money from a series of wins in major Group One races such as the Prix de L'Arc de Triomphe and the Epsom Derby.

"Sea The Stars commanded a $133,000 fee at the start, so it is not impossible that Frankel's first valuation will be around $155,000 and that may be a touch on the conservative side," McKeever said.

"There are no guarantees with this, and the performance of a horse on the course is not always a marker it can be successful in creating the next generation. But Frankel's achievements in racing, coupled with his bloodline, means he would be more likely to sire a good racing performer."

Photos: Racing horses take to Twitter Photos: Racing horses take to Twitter Tweet Lady – @peopleshorse: "Really enjoying my little holiday - getting a serious tan! NOL." Hide Caption 1 of 5 Photos: Racing horses take to Twitter Black Caviar – @blackcaviar2006: "Goodbye UK, and thank you for your hospitality Your Majesty. I enjoyed my time in Newmarket and racing at Royal Ascot xx." Hide Caption 2 of 5 Photos: Racing horses take to Twitter Frankel – @frankel_horse: "Thanks for all ur kind messages...hopefully the news I may run an extra race has put a smile on your faces." Hide Caption 3 of 5 Photos: Racing horses take to Twitter I'll Have Another – @Ill_HaveAnother: "Borrowed IPhone so I can write one last thing re: NYT and MSNBC pieces of shoddy "journalism" SANCTIMONIOUS. Now back to quarantine." Hide Caption 4 of 5 Photos: Racing horses take to Twitter Anfield Cat – @AnfieldCat: "so it turns out Andy Murray is infact Scottish, not British." Hide Caption 5 of 5

Photos: Australian super mare Black Caviar Photos: Australian super mare Black Caviar Galloping towards 20/20 – Australian superstar mare Black Caviar will make history if she wins her 20th race in a row at Morphetville on Sunday. Hide Caption 1 of 6 Photos: Australian super mare Black Caviar A winning couple – Melbourne jockey Luke Nolan and trainer Peter Moody will be hoping their champion throroughbred can continue her unbroken winning streak. Hide Caption 2 of 6 Photos: Australian super mare Black Caviar Hopes of a nation – Black Caviar fans hold a sign referring to Phar Lap, the legendary New Zealand-born, Australia-trained horse which captured the public's imagination during the Great Depression. Hide Caption 3 of 6 Photos: Australian super mare Black Caviar Zenyatta record under threat – Black Caviar is aiming to break the record set by American mare Zenyatta, who won 19 consecutive races before coming second in the 2010 Breeders' Cup Classic. Hide Caption 4 of 6 Photos: Australian super mare Black Caviar History's racing legends – Black Caviar has already beaten legendary British thoroughbred Eclipse. The 18th century horse won 18 races in a row. Hide Caption 5 of 6 Photos: Australian super mare Black Caviar Challenger Frankel – Four-year-old British colt Frankel is the only horse in the world with a higher rating than Black Caviar. Both horses will compete at the Diamond Jubilee Stakes, Royal Ascot, this June. Hide Caption 6 of 6

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Bloodstock valuations have come down from the crazy prices of a decade ago, when horses would sell for $10 million as purchasers found it difficult to secure a return on their investment with falling prize money at racecourses around the world. But the high end of the market is still very lucrative, with yearlings from top breeds selling at $2 million.

The statistics show exactly why a Frankel-Black Caviar horse would be much fancied on a racetrack.

Frankel commands a Timeform rating of 147, which is well above the average for a Group One race winner of 129. Black Caviar is the highest rated filly of all time, rated at 136.

"Ever since he was a three-year-old, Frankel has seemed determined to get better and better," Timeform's flat racing editor David Johnson told CNN.

"He had already rated at 140 by his first 2000 Guineas victory last year, and by the Queen Anne Stakes at Ascot this year he had overtaken Seabird as the greatest horse since records began.

"It seems a shame that his owners do not want to test him at the longer distances, and I think what we are possibly seeing now is a winding down of his attendances with the chance he will be at stud by next year. It is unlikely we will see him at the Breeders' Cup or the Prix de L'Arc de Triomphe."

Black Caviar, who turns six this month, may be retiring at roughly the same time. Trainer Peter Moody is still undecided whether she will get a few more run-outs as part of a farewell tour of Australian courses, or whether June's Ascot appearance was her last race.

The cult status achieved by both horses has spawned a lucrative sideline business, with everything from Black Caviar cufflinks priced at $42 to a $15 Frankel coffee mug available through online stores. Goodwood's recent sellout crowd was largely down to the attendance of Frankel and both have their own Twitter pages.

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Despite the desires of the racing industry to see these giants of flat racing procreate, there are two hurdles to overcome.

First is whether Frankel's owners want this to happen. There is a feeling in the industry at the moment that, come the day Juddmonte decide to put him out to stud, they may want him to pair with mares already in their stable so they get to reap the rewards of whatever he will produce.

However, that may be just for the first crop of the next generation, and they may be happy to let Frankel go to stud elsewhere later on in his career.

The second problem might be a little bit harder to overcome. The path of true love is not easy for humans, and it is no less an issue for horses.

"At the end of the day there is no guarantee that Frankel and Black Caviar will take to each other. They might not like one another, much like some humans are unlikely to mate," McKeever said.

"That is why, however much racehorse fans would like to see this happen, and however much money is offered to make it happen, the only two beings that can ensure it does are Black Caviar and Frankel."