I have seen them in Wally world for under 100 bucks.
Cabelas has a flint lock just under 300 bucks.
Muzzle loading kit
One of the diffferences to look for is inletting. Some kits are finsh and assembly, some require a lot of woodworking.
Before you buy, check your local laws to make sure whatever you build, you can use it.
Good luck and send pics when you are done.