5) Vincent Janssen (Tottenham)
Spurs paid £17m for the Dutchman in 2016 but, in his 43 appearances for the North London club, he managed to score just six goals (four of which came from the penalty spot).
Remarkably, Monterrey offered to pay £8m for Janssen to sign him on a permanent basis last summer, and he is currently still plying his trade in Mexico.