Julie,
You sound like my kinda collector...ecletic!
There is a very hard to read and understand price guide for Modern Library (if you need the source info, let me know, I think it's from Spoon River Publishing)...however, just like with all/most books, ML's have first editions (stated on first page), and only the really big names in great shape get the big bucks...The Maltese Falcon is one example (probably worth over $50 in grat shape)...however, some big names (like Hemingway and Falkner) don't go for alot becasue of the huge number published.
I'm spitballing here, but I guess if a ML has a dj and is a first in good shape...and it's a novel, they published many non-fiction books...$5.00 minimun. (Then again, "priced to sell" is a tricky business).
You might try your local library for the book At Random, the autobio of Bennet Cerf, who was the owner of Modern Library...a fun read.
Thanks for the reply,
All the best,
Gary