Sarge-

Good support for the relative likelihood of your finding a Tolley 8-bore is the fact that Tolley was a specialist in the large bore-rifle niche.

Tolley 8-bore rifles will value somewhat lower than H&H or Purdey, but not so greatly as to show up as 'bargain' priced guns.
Tolley guns and rifles are very much high quality product.
Another name you might shop for is Rodda.
They are in a similar value tier as Tolley and also built some very nice guns and rifles in the very large-bore niche.

Consider the very scarce nature of the 8-bore cartridge rifle in the first place, and that servicable surviving examples will command a very good price.

Further, the rifle shown above would likely need quite a workover before it would be a running rifle.

Gatsby noted a single barrel H&H which did not sell at a recent auction -- I'm not sure of the details there, but that might be showing that if you shop single shot rifles, you may get your prize at your price.

Also consider a muzzle loading rifle.
It's unlikely you'll find a SxS 8-bore in shooting shape much or any cheaper than a breech loader, but there are quite a few modern builders putting out beautiful replicas of 19th century work.
Look through SZIHN's posts for photos of some of his english sporting rifles.
He does 8-bore rifles that shoot very well and are quite beautiful.

--Tinker