I buy whatever is pretty and on sale. I don't think I've bought ANY workout wear at full price, except for some cheap stuff, but I prefer getting higher quality stuff on sale.
I've gotten some things from Walmart (a couple of $3 camis), Target, Dicks, but I also check out online sales.
I used to get quite a few things from Carushka during occasional sales, but since they've come back (they were out of business for a while, because the owner was ill) they don't seem to have as many sales).
Champion has a yearly workout bra sale ($19.95 each, for tops that are regularly $25-$40) and I picked up several this past year.
Someone posted a link to a sale at Paiva earlier (sale is over now) for up to 70% off already sale prices. I picked up some pants (to go with those tops!) that were regularly anywhere from $65 to $110 (!) each for about $15 each (believe me, there is NO WAY I'd pay $110 for a pair of workout pants!).
I've also picked up some sale stuff at
www.marika.com
I wish I could again find one particular online store I bought some weight gloves from, and ended up picking up some tops for $6 each. There was no choice beyond size and general style and color preferences, and they sold in groups of 4 (?). I picked some up and actually liked everything I got.
I will sometimes do a search for "fitness wear sale" and see what pops up.