I usually look for "openess to negotiate" players, those at clubs, and offer a player in trade. Those not at clubs, that don't have the "negotiate contract" option usually come for like 5,000 points. Check every season for "new comers" and "unaffiliated" players, ya never know who might get released from their club. Apart from that, I guess it's just down to luck if a big player comes for a less than the expected amount.
It's a pain in the... no one ever tries to buy the players I put on the market for a possible transfer, and I'm talkin' players with awesome stats who are not all that old.