You will have to pony up a few more bucks than the $400 you mentioned, but the Savage ML10-II is the best bet for you. You don't have to use black powder, but can use smokeless instead. No messy cleanup, just the same things you do to your cartridge rifle, plus cleaning the breechplug occasionally. No corrosive residue that rots the innards and ruins your accuracy and you also get higher velocity with less recoil than any BP muzzleloader. From Natchez they start at about $435 and you can get a stainless version for a bit more. Savage says they will get 1 1/2" accuracy at 100yd out of the box with the proper loads and I and most of the guys I know have found that to be true with their guns.

The Savage will work fine with BP or subs as well, but if you are willing to put up with the mess and the lower performance of BP you might as well get a NEF Sidekick for about half the price. Its a US made gun and the break open design is a lot easier to clean than most.