I believe if you leaf through Twain's Notebooks & Journals (which are well
indexed), you'll come across various entries Twain made about expenditures
as an adult, which he often wrote down.  That won't be perfect if you're
trying to correlate something in Huck's time with today, for example, but
it's a start.  I also highly recommend that you seek out a good business
librarian and tell them your question.  There are a number of sources that
provide statistical cost indexes going back quite some time.  They're
fairly unreliable before 1860, but that's half the fun of the search.  You
might also check out newspapers from the time and place in which you're
interested.  Many advertisers weren't shy about listing prices.

Scott Dalrymple