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. :)
No, it's not the only way. You can do research on your own to find out what common prices are. It's pretty simple.