Some of the key things I look out for, apart from the selling club are what the scouts say - they'll give a guide on the price. Also, keep a special look out for contract situation of young players - if someone is on a youth contract you can frequently offer a contract and compensation will be determined rather than making a bid to the club and it can be a great way to pick them up cheap (watch that sometimes it can be more than they're worth anyway).
Make sure you know the dates for the regens - with those you can offer a contract before the club who brought them through and steal them. It's more risky this year with the reduced scouting information but can still be a useful trick.