Interesting question, for which I don't have an answer. But I do have an overview of stuff to be careful about when considering a contest. It started at a screenwriting site but is applicable across the board to all writing.
First, check out what it takes to submit. Is the entry fee reasonable ($25 to $50)? Is the prize money at least 100 times greater than the entry fee? Are there any extra fees for consulting, critique, mailing or delivery beyond the initial submission, editing, photocopying, etc.?
Does the contest promise what no contest can deliver: guaranteed sales or options, connections, internships? (These things can come from doing well, but it's never a sure thing.)
Are the judges people who are employed in the film industry or publishing business? Is each well qualified to judge the work of others? What are their credentials?
Read the terms and conditions for submission all the way through, no matter how fine the print. Make sure you understand every clause. If you don't, you could be giving up some of the rights to your own work.
Beware any contest which does not have a street address (not just a post office box) and a listed telephone number; which exists only online; which does not share the names of past winners and offer proof they received their prizes; which does not share the names and credentials of judges; which feels 'fishy' and seems too good to be true.
Remember, too, that there's some logic in thinking that if your writing is good enough to do well in a major contest, it's probably good enough to be marketing to buyers.
Maryn, who enters few contests