Noble Knight Games slashed the prices on all of its Scarred Lands stuff a while back, as did White Wolf. I'm not certain if that is still going on, though. Amazon prices were as low as $7 for almost all 3.5 books last fall, though since then, many have joined you in your search for bargains which has caused both Amazon price and eBay prices to rise. The surge in people wanting to buy D&D 3.5 books a few months back caused a price spike.
I honestly think that the window for 'dirt cheap' D&D books may be closed (at least until interest in the books subsides again). You're going to have to pay close attention to special sale announcments (e.g., the ENWorld GM Day sale) to get a truly good deal. There might be another window for 'dirt cheap' after the physical rlease of 4e but, until then, so long as many people are scurrying to snap up 3.5 books at 'bargain' prices, you can expect any truly good deals to be few and far between.