For me, PetCo is a few miles further out, but PetSmart has 20g's for $32 or so. And while $20 would certainly be cheaper, it's not a lot of money.
The reason we talk a whole lot (seriously, an incredible amount) about the $1/g sales at PetCo is primarily 40g breeders, with a secondary emphasis on 55g's is because it's about a 50% discount (maybe more). For the standard 10-20g tanks, you wouldn't expect to spend a whole lot more than $1/g at normal sales prices.
If you're very thrifty, sure, wait till the sale and go crazy! The same advise goes for buying multiple tanks. But for a single small tank, I'd just go to my nearest chain LFS and grab it, knowing that it's a pretty good price even when not on sale.