Last year it wasnt that bad at all, not that dissimlilar to a freezeout to be honest. Think there was about 2000 entrants. It all depends on the perceived value really. If you consider yourself capable of a top 30 spot then you could consider maybe 5 rebuys. If you think you are going to be lucky to get into the top 200 then maybe just 1 rebuy. In short, Be frank and honest about your prospects and decide your rebuys from there.
EDIT EDIT EDIT...doh....it's a feeder. This makes it even easier. If you think that your abilities and chances are good to qualify (530 value), then you can attach a hypothetical % to it. eg you think you have the patience and ability to bring it home, then if you think you've got a say 5% liklihood of doing it, then the perceived value is 5% of 530 ie $26.5. Therefore dont spend more than this on rebuys and add-ons. Adjust the % realistically according to your ability and how you stack is looking compared to others.