1943 0-18? If it is indeed in "very good original condition", ie,

1. original finish intact, with perhaps a bit of playing wear
   
2. no cracks in top or body (or cracks have been properly repaired)
   
3. comfortably playable action (distance from strings to fingerboard) without buzzes.
   

...then Jeff's estimate is probably pretty close, but if you're looking for a quick sale, you may settle for a little less. One thing you should realize though is that since Martin guitars never until recently had adjustable truss rods in the neck, they at some point require a don't-try-this-at-home professional neck reset which MUST be done by a very skilled guitar repair professional. It costs a few hundred bucks and considering its effect on the value--both financial and musical--of the instrument is well worth it.   

Before you do anything rash, though, consider that that's a family heirloom of great intrinsic worth. If you have any family members who play (and are respectful of good instruments), why not offer it to them? It's a fine instrument but money-wise, you're not looking at a fortune here.

Note that the correct designation is 0 (zero) even though we call them "oh-18" in common usage--searching for a zero 0-18 will get you results that letter O-18 won't (although they are often listed as letter-oh). Right now Mandolin Bros in New York has a 1936 0-18 which you should have a look at by comparison: see http://www.mandoweb.com/1_Acoustic.htm

The 1936 is priced at only $3500 (prewar Martins are worth more than post-war; 1943 is transitional) as a "player" rather than "collector-grade" vintage instrument.

Good luck and I hope this instrument finds a good home, or stays with you.

W-O