This doesn't have anything to do with cost, but a friend of mine who is an avid collector and African hunter has some nice pictures of a variety of 8 bore rifles from several makers to drool over at:

8-bore pictures

In addition to muzzle loaders, you might want to keep an eye out for a 10 or 12 bore double too. Eights command a bit of a premium these days and a similar actioned/condition 10 or 12 is often much cheaper, and still a hell of a pig gun. They are also more common. I found a 10 bore Tolley at a "reasonable" price a few years ago because it had non-rebounding hammers and someone had blacked the damascus barrels, neither of which bothered me the least as it was mechanically sound. It will be my moose and bear gun this year.

My final advice if you go with a bore gun is to save those nickels and dimes, find a gun you really like and get it checked out by a pro, and then just do it. They're highly addicting and they are not making any more of them. You'll be glad you did, trust me! Keep us posted on what you find.

Bob