Yes they are a lot of fun but there are other expenses associated with using BP guns. Most of the stuff is cheap individually but it does add up. A lot of it isn't needed but it makes shooting and cleaning much easier. Powder measures, capping tools, anti seize(for the nipples), grease, lubes or lubed wads for shooting, bullet pullers.nipple/flash hole picks etc. I have a whole box full of stuff and it can be a pain to drag everything out to shoot. Cleaning is a must when you're done. You can't go shoot and set it on the bench and think I'll clean it tomorrow or the next day because you'll come back to a rusty mess. If you dive into this buy real BP and not a substitute. I used pyrodex and after reading stuff on here and talking with a local guy i found that it cleans easier and is less likely to give you a malfunction. Hot soapy water is the best way to start the cleaning, it will remove 80-90& of the fouling rather quickly and then i just clean like a regular gun after. make sure it's dry and oiled when done and it's not a bad idea to go back and check them a few days or a week later to see if you find any rust....which it will do if you miss a spot. Not trying to deter you, just a warning. BP is super fun to shoot but after all the work you'll find you won't shoot them all that much. They are also addicting too,i think I'm up to 11 or 12 between revolvers, rifles, shotguns and muskets, a few are cartridge and the rest are front stuffers.
If you don't mind buying used you can pick up many BP revolvers and rifles in the $75 to $150 range too.
Oh and the one Mad posted above is a replica gun which means it's non firing, repros are generally a functioning gun so if you order one online it's something to watch for to make sure you're getting what you want.
Another thing to watch for is the difference between brass and steel frames. Brass revolvers are perfectly fine but the powder charges will be lower than steel frame guns, usually around half so make sure you check that difference when looking up loads. Also you CAN'T use a cartridge conversion cylinder on a brass frame gun.