Judges should, at the very least, be professionals in writing or publishing. Moreover, respected judges add weight to your potential win. Most contests rely on the sale of anthologies to pay for expenses, but some subsidize their prize with an entry fee. Even the smallest contests pull in 20 to 50 entries. If, at 50 entries, they would be earning ten times the prize they're paying out, send your work elsewhere.
Prizes vary greatly from contest to contest.
A contest that offers only publication is suspect. A small contest offering publication on the web site will not be much of a credit, and there's a good chance that the coordinators are getting money from advertising revenue. A contest that offers publication in an anthology but doesn't give contributor copies or royalties is also suspect. Cash prizes should be proportionate to entry fees, and payable within a reasonable amount of time. Under no circumstances should it be the winner's responsibility to collect the purse in person, or pay to receive it.