I think for ease of agreement, we should stick to Transfermarkt, unless there is no alternative but to use another source.
Bergkamp is a good example as Lobo says because he's on some other sites as around £9m.
Keeps us all honest and keeps it devolving into arguing about which source is more reliable. I've literally only found one player I've an interest in with no fee listed on Transfermarkt. Nowhere seems to have that info in all honesty.
Betty - if it's not there or on Wiki, I'm sure we can all agree on an alternative source so pick the player then if anyone has any remonstrations about it you can disclose where the info is from.