I second the Tires23 recommendation- great prices, and if you find them lower, they will typically match whatever price you find.
I've purchased Yokos, Hankooks, and Generals from Billy and have been extremely pleased with all of his recommendations.
If you're going 225/245 staggered, there's lots of choices that should come in around the $600 price point. Depending on how you drive and what you're looking for, consider the following;
1- Yoko S-Drives; good dry grip, outstanding wet traction, predictable breakaway, no tramlining
2- General Exclaim UHPs; a little squishy buy quite grippy, great treadwear, excellent ride.
3- Kumho SPTs; see previous post
Dunlop Star Specs; these are the go-to tire for most of teh guys on M3f.net; they are cheaper and better than the PS2s that the M3 came with. No experience with them personally.
SOLD- Now Enjoying Honda Civic