Alex Song
When: 2012
How much: £15m
The Cameroonian turned himself into a lynchpin at the Emirates, and bossed the club’s midfield in a way that nobody has managed since. Barca came knocking in 2012 and paid £15m for the privilege. Like Petit, though, he struggled to become a regular fixture at the Nou Camp thanks to the club’s already crowded midfield, and managed to make just 65 appearances in all competitions during his two-year stint in Spain. Still technically on the books, he joined West Ham on a season long loan in the summer.