7. Hernan Crespo
Bought for: £16.8m by Chelsea in 2003
Sold for: Released on a free in 2006
Argentine marksman Hernan Crespo can count himself rather unfortunate to feature on our list of costly transfer blunders, but simply failed to convince Blues boss Jose Mourinho that he deserved a long-term position as number one striker at Stamford Bridge.
Crespo arrived in West London with a big reputation earned by hammering in goals from all angles at Lazio, but despite a very favourable goal record with the Blues was shipped back to Serie A with AC Milan just a year later.
Eventually after a year back at the Bridge the forward rejoined Inter Milan on a free in 2006, but has still done enough to be remembered fondly by some Chelsea fans for a series of clinical strikes.