For normal folk, window shopping costs nothing, it is spending in the mind, while the January sales are an opportunity to hunt for bargains. But in the looking-glass world of football the January transfer window finds clubs paying inflated prices for players they may or may not really need. On balance the sellers tend to end up better off than the buyers.

The window was introduced by Fifa in 2003 following negotiations with the European Commission over the labour laws. A similar system was already being used by a number of European leagues, although in England the window replaced the previous deadline, usually the third Thursday in March, and some still mourn its passing.

Steve Coppell has called for the New Year window to be scrapped, saying that it leads to panic buying. "I cannot see the logic in a transfer window," he argues. "It brings on a fire-sale mentality, causes unrest via the media and means clubs buy too many players. The old system, where if you had a problem you could make short-term purchases, was far better than the system we have at present."

Steve Bruce may be inclined to agree. In the middle of a successful season, with his Sunderland team pushing for a place in Europe, he has suddenly lost Darren Bent, his leading scorer, to Aston Villa. Not so long ago anyone offering £18m for Bent would have had his hand bitten off at the elbow, but goals alter cases.

The Midlands club may end up paying £24m for the striker, which would bring the total amount spent on him by Charlton, Tottenham, Sunderland and now Villa to £53m, making Bent the costliest British player.

Not the most valuable, of course, bearing in mind that Ryan Giggs and Paul Scholes cost Manchester United nothing but the money spent on bringing them through the youth academy, and have long since been beyond price in their worth to Old Trafford.

For Villa, spending that much on Bent looks like a panic buy. Martin O'Neill's sudden resignation at the start of the season was put down to a difference of opinion with Randy Lerner over the Villa owner's sell-to-buy policy. O'Neill was promising to take the team into the Champions League but under his successor, Gérard Houllier, Villa are now in a serious struggle to avoid dropping into the Championship. So they end up paying out more for Bent than O'Neill would probably have wanted to spend last summer.

At least Bent should prove a happier signing for Villa than an earlier January buy, Eric Djemba-Djemba, who having already underwhelmed Manchester United after Sir Alex Ferguson signed him from Nantes was offloaded on deadline day at the start of 2005 and spent most of his time on the bench. Such was the fate of Scott Parker, for whom Chelsea paid Charlton £10m a year earlier. Parker's career stalled at Stamford Bridge although he would probably get a better run in Carlo Ancelotti's midfield now.

Not every January signing is a flop. Newcastle may still wince at the memory of Jean-Alain Boumsong, an early candidate for the sale of the 21st century when he left Rangers for £8m six new years ago but another defender, Michael Dawson, who joined Tottenham from Nottingham Forest at round about the same time in an £8m deal which also involved Andy Reid, looks a better bargain by the season.

Similarly Matthew Upson, who moved from Birmingham to West Ham in the 2007 window, has been good value for his £7.5m fee although not quite the snip that Gary Cahill, now an Arsenal target, has turned out to be at the back for Bolton, who signed him from Villa for £4.5m at the beginning of 2008.

Unless Bent proves otherwise, the best sell of the January windows so far is arguably Craig Bellamy's departure from West Ham to Manchester City for £14m in 2009 while the shrewdest buy is surely Dean Ashton's £7m move from Norwich City to Upton Park in 2006, which did much to help Alan Pardew's side reach the FA Cup final.

This week, incidentally, marked the 50th anniversary of the day the £20-a-week maximum wage was abolished in the Football League. Up to now Aston Villa have had a slightly more generous weekly wage ceiling of £65,000 but Bent's salary is set to go through the roof. And he experienced fewer problems getting away from Sunderland than George Eastham did with Newcastle.

Bent just went.