$10 for a short, firm foam roller. This is the one I bought for my personal use. It is easy to travel with, so when I go on the road and will be at commercial gyms that probably don't have foam rollers, I can take it with me. I've had it for about 2 years now and it has not broken down yet.
If you check in with Elite FTS, they will sometimes run specials with free shipping so you won't have to worry about that. Honestly, you are not going to find one cheaper than that at Walmart and the quality will be MUCH shittier at walmart.
You could go with BIrd's advice of the PVC pipe, but I have found the pipe to be too aggressive with certain body parts (especially if around a boney landmark), especially if you are not used to the rolling.