I haven’t tried the Smart Cover compatible versions, but I’ve been very happy with the soft TPU back covers I’ve gotten on eBay for around $8 with free shipping.
Here’s a clear one on amazon, but I’ve seen them on eBay. If you’re willing to wait weeks for free shipping from China, you can pay even less.
I have a clayco frosted one from amazon but the frosted layer is peeling after less than 2weeks... but it’s cosmetic so I’m closing onr eye abt it.. just saw a tech21 one on the apple website for $60