These are so cheap and fast to make you can give them away with every purchase. Customers love getting something extra they were not expecting. If you pay full retail in a craft store for the hardware you might spend 60 cents per key chain. Buy online and you'll pay half that.
I have included a pattern book with a few key chain patterns. These are really just to give you the idea. Make up your own. You can't find an easier scroll saw project.
These key chains can be a little fragile if you are not careful. I use a 1/4" split ring to connect the chain to the wood. If you just put the chain through the hole it's a little stronger but the ring looks more finished. When you drill the hole for the ring leave as much wood around the hole as possible. You can also super glue very small copper tubing in the hole for strength.