Regarding Modric's fee, the 33m you say is what I've been reading too...but that's been in Euros. Transfermarkt have it down at £26.4m and they're generally very accurate.
If Spurs don't push on and kick on, they cannot dictate the price fully. If it was a case of Spurs selling Bale tomorrow, yes claiming £40m would mean I live in cloud cuckoo land, but I've already said I expect him to stick around for a bit and eventually when he's stated he wants to leave to actually achieve something other than the 3rd/4th placed trophy (Wenger, 2012), you guys probably won't be able to fully dictate the price.
However to a PL club, you'll have far more power in setting an extraordinary premium than you would to a club like Madrid, especially whilst having a player who would want out.
This is all hypothetical. If Manchester United can go to Arsenal and sign their best player, anything can happen (contract issues or not).
It's certainly not a case of the best player in the world who has a European cup medal in his collection going up for sale though, £40m to a foreign club seems very plausible to me. Unless £40m has suddenly become the rate for a 'decent' player?