My daughter has done really well with the Target brand formula. We tried Costco, but it gave her really bad diarrhea, which led to the diaper rash from hell, so back to Target! Too bad, because Costco was so damn cheap.
We got a bunch last month when they had the "spend $100, get $25 gift card" promo, and should be set till she turns 1 next month and transitions to cow's milk. Using that kind of promo, plus the Cartwheel app, definitely saved us some money.
She's still partially breastfed, too. Keep in mind that breastfeeding doesn't have to be all or nothing, if you're struggling. Both my kids started out 100% breastfed, then with my son, not being able to pump at work for a while made me lose most of my supply, and with my daughter, I intentionally cut down because she would NOT stop pinching and biting me while feeding. So if you can and want to keep breastfeeding part time, that can save you a little cash. :)