Amazon puts them on sale once in a blue moon. About four months ago, every season of Superman was 14.99. I realize that's not helpful now, but if you're not in a rush, they'll probably go on sale again.
I've seen (and purchased) BTAS at EB/GS. It's not too difficult to find them, and all stores have the B2G1 or B2G2 on DVDs going on now. I've seen all three volumes together multiple times.
Wherehouse typically carries them too. Regular price + shipping is not so good, but their specials change weekly (on Fridays), and they seem to be fond of the 20%-25% off DVDs special, which seems to happen every 3 to 4 weeks.