Rare?
Well, according to Blue Book, Sharps converted 31,000 carbines and 1000 rifles from percussion to .50-70 cartridge in 1867; and Springfield Armory did 1000 more rifles and 300 carbines in 1870, and I am sure a lot of them were worn out or lost. Book value is considerable, but I don't know what they are actually selling for, see below.
The three groove barrel shows that the original six groove percussion barrel was larger than .5225" and was relined to the same specs as the Trapdoor Springfield.
There were many fewer rifles than carbines converted but Blue Book says that does not make them worth much more. I don't know about THAT. It will be interesting to see what the guy brings in.
Worth the time for what?
Buy low, sell high?
Check your market, there are two going begging in the $1000-$1200 range on GunBroker, which is way under blue book numbers.
Shoot?
Ammunition is available but expensive. Handloading is like any other BPCR which is a more involved process than cranking out a few .223s for your tackytickle asssault riffle.
A friend has a carbine, it is about Minute of Apache accurate with a good stout kick to let you know you have fired a serious weapon.