Motherhood looks expensive, but they have tons of coupons. Every now and then they have a clearance sale that is awesome. I got a ton of maternity clothes from them for less than $100. Old Navy is also pretty good, but I hate that the store near me never has any maternity things in stock. I've had to order two sizes of things and return a lot to make sure that I got stuff that fit, and that was annoying. Khol's is also not bad when they're having a sale.
Old Navy and surprisingly Wal-mart does too. For Wal-mart, you have to shop online, but they actually have a lot of different maternity clothes and maternity bras.
I have had the best luck with Motherhood, Gap, and Old Navy. In our local stores, Gap and Old Navy never carry maternity clothes in the store--so I order online.
Goodwill! I've gotten Motherhood and Old Navy clothes there in great condition, and cute stuff. Always cheap. I usually pay less than five bucks for a pair of jeans--even better if it's on a tag sale!
Re: Maternity Clothes.
I've gotten clothing from others (sister-in-law, friends) and bought from Old Navy (online) and Motherhood Maternity.
I got a few things as gifts from Amazon, but it didn't work out.