Everything varies in price and metal quality. I feel like the cheapest cage that I noticed was around $20, but I wasn't looking too hard at those. I ended up getting twins, so my ceremony was a little longer than the people behind me who got a single pearl. They told us to come back in an hour so they could set my pearls in the pendant I chose, but if you just choose a cage, you wouldn't have to worry about that. Oh, and the chain is a separate cost from the cage. If you end up with a setting that you can't immediately wear (my pearls were attached to the setting partially with a glue, and they said not to wear it for 24 hours), they charge 99 cents for a small cardboard jewelry box to put it in too, LOL. So, we paid for: the pick a pearl experience, the chain, the twin pendant setting, and the box. I wanted to mention that so you'd be prepared!
It's a fun experience, and I'll probably do it again on our next trip!