If all you are going to do is shoot 100 yards or less, about any of the inlines on the market will do that with the right bullet combination. If you want to shoot some extreme distances, while the cheap ones will still do that, it might take a little more work.
The idea of a cheap muzzleloader to me means one that has a poor fit and finish of the stock and hardware. For instance, I have a $89.00 CVA Staghorn Magnum I bought years ago. Out to 100 yards with a 245 grain powerbelt of a 250 grain Shockwave, it will shoot with the best of them. But the gaps between the barrel and stock might shock some. Yet it just flat out shoots.
Check the auction sites and the forum classified adds. There are always rifles turning up that are not as expensive and most and are still good shooters.
If you want new, look at the CVA Wolf, T/C Impact, or some of the Knights that are rolling around.