People seem to forget that purchase value isn't directly related to ability. No point whatsoever in comparing the value of 2 players. It's a mixture of:
Time left on contract
Which club they play for
Nationality
Age
Whether the club actually wants to sell the player
Financial situation of both clubs
Wage demands
etc.
At the end of the day, all a transfer fee is, is essentially compensation for taking someone's player whilst they are under contract.