When it comes to iPhone cases, you don't get much cheaper than this. Depending on what color you get (yes, it's available in multiple), you can pick one up for around $7-$8 shipped (or less if you're an Amazon Prime member). Needless to say, this one's about as basic as it gets, but for a lot of folks that will be fine.
Note: MiniSuit also makes a few other variations of this TPU case but they are all very similar.
See pricing for the MiniSuit Frost Matte TPU case from Amazon.com.