A scope I use a lot on muzzleloaders are the Nikon Pro Staff in 2-7x32mm. They are a shotgun/muzzleloader scope. They are also Nikon so the life time warranty is always there. Because of the 2x they make a great stalking powder and with the 7x you can do any kind of field shooting you need. The Pro Staff gathers light great, has excellent glass, is tuff as nails as I shoot large power loads and have yet to shake one apart. Also they are not expensive by some standards. You can get them for $140.00 or less. Watch for them on sale and I picked one up for $89.00 once. Also they have great eye relief and are only 11-1/2 inches in length. That means they fit all rifles and normally the rear sight can stay on the rifle. So if you mount them with quick release rings.. you have a double aiming system. This was one scope that was recommend to me by a person that competes in target shooting matches and owns a gun shop. I asked him to recommend a scope for my White Rifle. He picked the Pro Staff. He said I could spend more but never get a better scope for my dollar.
Natches.com was selling Nikon Omega 3-9x40 for $149.00 a week or so ago. Check there and see if they have any left. That is an excellent scope as well. For all the above reason.
If you really want to save some money and do only 100 yards or less shooting, I have used a Simmons Pro Diamond 4x on the scope. Granted it is a $39.00 scope. But the Glass is good, clarity, seems tuff as nails, and do far has done all I asked for it.
Another good lower end scope is the Bushnell Banner Dusk to Dawn 1.5-4.5x32mm. Again low cost, good glass, excellent clarity and light gathering ability.