Quality parts to build a muzzleloading rifle with a single barrel, single lock, runs $1,000.00 to about $1,300.00 today, whether longrifle or 1/2 stock shorter plains-type rifle.

A pair of locks, double barrel set and the wood, would cost more, if it was available from an outfit like Trackofthewolf or one of the others - Petatonica, for instance. Those parts are just not available.

The condition of the parts has "some" value in itself(knowledge).
That parts "kit" sounds quite reasonable, to me, if the locks and barrels are in good shape or new.

Since the barrels need to be joined, are ribs included in the "Kit"?

I assume the barrels are breached? What about the tang? included?

WOW - yes, an excellent price if of decent condition & all parts needed, are included - hooked breaches and matching tang.

This is not a "kit" for less than a really good builder, someone with experience.