Realistically, in your opinion, his value is closer to 25-30 in an OPEN market.
Its not an open market, there aren't numerous Modrics to choose from, there's one and the price for him as a player will be set by the club higher than comparative players or sales of recent times.
As I see it, and this is my opinion, his valuation will be above that of a Henderson ie £20m, and below that of a Kaka (as previously stated £56m.)
If current media speculation is to be believed, the red Mancs are currently lining up to launch an opening bid for Sneijder of £35m, comparable player, same position, same importance to his current team, same value placed on getting hold of him by the prospective buying club. Now I'm not saying that Modric is on the same level as Sneijder, but I am saying that the effect of losing them to their current teams would be the same so if the red Mancs get Sneider it'll be for at least £10m on top of the opening bid, if not more and the same will apply for Modric. £32-35m would probably be an acceptable bid, in the current market.
The difficulty with that value though is that you now have a situation where relations between player and club have broken down. Short of demanding a transfer request (which Levy has intimated he will also disregard) Modric's only other choice would be to go on strike like Berbatov did. If it wasn't Modric (or those around him) forcing the move, then Spurs could realistically ask for silly money. But it's a situation that's degenerating fast, if you end up with a player who refuses to play for you, he is effectively worthless. So it may be more pragmatic to accept a bid closer to £30m
