That is really a hard question for a number of reasons...

What is a fair price to you?
What State laws pertain to the style of muzzleloader?
What projectiles do you plan to shoot?
What kind of ranges are you expecting to encounter?
Are scopes legal?
Do any styles interest you more then others?
What kind of skill level do you have with muzzleloaders?
How many days a year do you intend to use it?

I am not trying to make this personal, but if all you want is to hunt five days a year and shoot out to 100 yards at deer.. a CVA Buckhorn Magnum for $99.95 will do you just fine. Actually this rifle would probably do it all for 99% of us. If you want a rifle that will be something you hand down to family, hunt and shoot a lot with, and want to really have some range time.. How about an Omega Z5. If you like Traditional challenges, a Lyman Trade Rifle might be in your future. All of them will do the job.

Some rifles I like...

Omega
Knight Rolling Block
Lyman Great Plains Rifle
Black Diamond XR
Triumph
T/C Hawkins

Others will I am sure have their own choices...