Trapdoors,,,,as of now ,,,are the one gun you can still buy a GOOD shoot-able original cheaper than you can buy a reproduction. I bought a very, very pristine 50-70 , 1869 TD for 500 dollars. Granted, I was a wee bit lucky to have a good friend that is a foremost athourity on trapdoors scouting from me one, but if you want a 1873 or an 1880's model,,and are careful and diligent in looking, you can beat reproduction prices,,and still have a piece of shoot-able history.
You will probably pay more for a carbine, but you can get a rifle and convert it.