a transfer compare tool would be the only way of not overpaying/underpaying for players. this way we would all know what to expect to pay/receive for a certain player. until then, i would honestly pay as much money as i have on a player, if i feel that the player would greatly improve my team. :)
but, yes, i think 2.5 mil is kinda too much, but that also depends on how good/young the player is.