IIRC, Lefever was pretty much the inventor of the hammerless double. They were bought out by Ithaca after some fits and starts in business. Some of the guns have considerable value, but the Nitro Special isn't one of them. It should be a respectable shooter though, if you have it checked over by a gunsmith.
A friend of mine had one in 16 ga a few years ago that I researched for him. Seems like the value was around $350.
EDIT: I think OOL was saying that the site in his link is not an auction site, not referring to THR.