A clause in the midfielder's contract, stateing that Everton will recieve 15% of the player's next transfer fee, means Everton can effectivly get a 15% discount on the player's actual value.
Sunderland, as clubs who don't have a pot to piss in often do, value him at a massivly exagerated £2m (anyone remember Mart Poom, and the £7m Derby wanted?). However, in reality he is worth no more that £1m, and Everton are considering a possible £500,000 bid.