Around here, ANY T/C guns have outrageous prices on them--not to say that they all sell for what the owner wants though...
I've seen REALLY nice Senecas go for as low as $150 from a more reasonable dealer... whilst another dealer about 30 miles in the opposite direction wants $275 for a fair looking .56 Smoothbore Renegade and $375 for just an average looking .50 cal. Flintlock Hawken.
Yet the consumer demand for the T/C guns is NOT that high in this area to command AND sell at those higher prices. Makes no sense at all. I even found a hidden treasure once in a dirty looking .50 cal Renegade that the dealer "thought" the barrel was rusted up too badly. Sold it to me for only $75. It was worth that much for parts alone! I used some very strong means of removing rust and oxidation and it turned out to be about a 95% condition gun and the bore shined up like a mirror!!! Doesn't happen often but it's possible. I ended up selling that gun for a really decent profit and bought many nice Christmas presents for the family that year with the proceeds.
The .56 cal. Smoothbore Renegades (if bought cheaply enough) make a great platform for a Green Mountain drop in barrel change. For about $150 or so, you can have a brand new GM barrel replacement. Figure $150 for a gun and that's only $300 for what should make a nice (though somewhat heavy) muzzleloading gun. Hard to find a Hawken, Renegade, or Lyman in superb conidtion for that price...
But I digress too much... Getting back to your original query about the Seneca and the Cherokee rifles. I see the same kind of pricing on auctions $350-500 depending on condition. I wouldn't pay that for one if it was still NIB, AND had the extra barrel in the other caliber (well... I might think about it a lot though) Most traditional styled muzzle loader users/shooter/owners tend to do things for themselves. It's in our nature to find a logical way out of a tough situation, I have no doubt that you will come up with a similar type of rifle without paying those exorbitant prices. AND, you wouldn't worry about NOT having replacment parts for a gun. But... we make do, NOT doo-doo...
I will keep my eyes open for a reasonably priced, small caliber, lightweight rifle for you. They do exist, but "seem to be in hiding" to the average looker. I have a fellow I can call this afternoon to check about it in fact.
Good luck on your search, and if there's anything else you are looking for, let me or someone know.
WV_Hillbilly