I use an airsoft Randall M1892 for my Steampunk carry piece... the gun (when you can find it) runs about $350, and the holster was another $75 (including fake cartridges)... so, not cheap... but lots of fun...
That, again, is Zoe's gun from Firefly (and other places)
Lever Action is great!
I also have (in my "real" guns) a Model '12 Remington in .22 WRF... it's a pump action, hexagon barreled beauty. Mine is an original (it doesn't SAY Model 12 on it, later ones did because it was first manufactured in 1912) The cartridge it is chambered in is larger than a regular .22 in length, even larger than a .22 Magnum... and this particular gun has iron sights on the barrel, and is scary accurate for hunting or plinking. Unfortunately, it is also very expensive to shoot (and not reloadable, as it is still a rimfire)
I really like the Mauser pistol, and have thought of it for a great build point for another carry accessory.
Also, a non-period piece (and mods feel free to delete this part if its too far outside) is the CZ61 Skorpion machine pistol... it screams "steam ME!" and there is an airsoft version of it available to do just that, as well.