Bumpity -- I'm slowly starting to scout around for a villa to host three generations of patokals in July/Aug 2011, and Samui is near the top of the list of possible locations. (Phuket/Krabi are also options, but it'll be rainier on that side of the peninsula.) Our budget's a bit less than the OP ($200-ish?), but we'd be staying for two weeks or so, so there should be more room to bargain.
We stayed at
Alam Warna in Seminyak, Bali earlier this year (only for a few days though), and it pretty much hit the spot: not just two bedrooms but two entirely separate buildings on one property, with a private pool between them. Any ideas for something similar on Samui?