I doubt if anyone would give you an estimate without seeing the rifle. But, if you have any experience with rifle building then you might be able to handle it yourself. The broken tang is no big deal. It can be welded with mild wire so it will take a finish. I'd have to farm that work out. The big "ifs" would be the size of the lock mortise relative to a new Siler. If the Siler's too small, then a "gunmakers" lock with oversized plate would have to fitted to the gun and that's a lot of grinding and filing. You might find that another style lock would be easier to fit. AND the big problem could be the location of the center of the pan relative to the touch hole in the vent liner you're going to have to put in the existing hole for the percussion drum, assuming it has one rather than a patent breech. Either way, you may have to have the barrel redone with a flint type patent breech to get everything to line up. Been there, done that. The double set triggers probably mean lengthening the plate mortise besides drilling /tapping for a new tang bolt. Thats not a big deal since you have the original hole through the existing tang(if you don't need a new one as part of a new patent breech) and stock as a guide for the drill. Setting them up for function can take a lot of time.
I found, after doing several projects like this is that it's very important to think before you do something that's not reversible, and about the sequence of the steps involved. If you decide on giving it to a gunmaker, you'll have to decide how valuable the rifle is to you relative to the cost of the alteration.
If worst case scenarios apply, it's not going to be cheap.
Sorry for the long winded reply.
Duane