As with most collectables, condition and correctness influence the price. So do variations within type. Over on
http://www.gunsamerica.com there are 1917's ranging from $475 to over $5,000 (a USMC model in 99% condition).
I would say that a serviceable shooter is going to run at least $500 or so. Refinishing or other collector's value-diminishing things might allow you to pick up a bargain (from a shooter's standpoint) from time to time.
Rosco