We almost need a picture so we can evaluate the worksmanship of the stock and the overall appeal of the rifle. Is the stock old or new? Is the Barrel a rebore of an original barrel?

As a shooter figure the cost of a new CPA or Ballard reproduction set up the same way as a starting point. I would also use the CPA or new Ballard workmanship as a standars. They are very good if this one is better it's a plus. Less would be a negative re-sale wise. 

All the extras are expensive too. That muzzle loading set up while intresting is not often seen at matches.  Not sure if someone would pay much more for those extras.  Sights that go with the rifle may make a signifigant difference in value too.

As a collector it may be another thing, but I am reading from your post it is new work done in the old style.  Sky is the limit if it's origional.

Boats

That Mike Stevens rear sight is worth at least 500 bucks, I sold one several years ago for nearly that, Axtel front is not cheap either Guessing they are nearly 200 dollars.

I really think it would price about the same as a reproduction of the same quality fit and finish.  To get a CPA together with finished stock bullet mold sights and all accroutments is nearly 5K.  Ballard possibly more. Of course if buying new or getting a gunsmith to build you get it set up exactly like you want

I know I passed on a good CPA when looking for a 2nd one some years ago, it was selling for 90% of new price, with a so so stocking job. Had it been half or 3/4 of new price may have bought but that close I went for a new rifle instead.

Not trying to be a smart alex just setting some parameters for you to make up your mind.

Boats

Ken Bresien made excellent barrels, and I believe, very few false muzzeled breachloading barrels. He started out making false muzzeled slug guns and later got into breachloaders. His slug guns were "second to none". I believe you have a treasure. Will see some guys that know him well this weekend and
ask some questions.

I would definitely not part this gun out!