Charlie Buchan - £100 a goal

 In 1925 Arsenal wanted to sign the veteran Charlie Buchan from Sunderland in 1925.
Based on the belief that Buchan was good for 20 goals in a season, Sunderland valued him at £4000.
Arsenal proposed that if Sunderland were that confident in Buchan's goalscoring potential (he was 34 years old) that they would pay £2000 for him, but add £100 for every goal that he scored.
Having rejoined the Highbury club, Buchan scored 21 goals that season, so Arsenal ended up paying £100 more than Sunderland had originally asked!