If you want to shoot really cheap, get a sidelock with a slow-twist barrel and shoot patched round ball. Get 100 balls for $12, use some "pillow ticking" from a fabric store for next to nothing, and get a tin of 100 #11 primers for $6 and go shoot. Only way to go cheaper is to buy a mould and start casting your own lead.
Unless you're looking for a 200 yard rifle, they'll do just fine out to 100 yards and put a deer down just fine with good shot placement, and you can't beat how fun they are to shoot.
Otherwise, get yourself a decent inline and some Hornady XTPs. The Hornady's are fairly cheap and very effective.
By the way, a modern inline muzzleloader is not "old school".
Regardless of what you shoot, just have fun shooting it!