I won a $75 contest awhile ago, after a few weeks of waiting i ended up only getting paid $25 because apparently "he didn't need the logo anymore"
Yeah that is an example of an unacceptable transaction, and i'd have reported the client for that - all you can do really! It's not your fault they didn't need the logo anymore. I guess you just have to put that sort of thing down to experience and hope that not everybody in the world is devoid of morals.
That is the problem with contests. Its massively underpaid. I agree with an earlier post that said about people in the USA, UK, Germany etc find no reason to enter these contests other then portfolio building, of which I'm doing at the moment. For me, having a chance (as that's all it is, no guarantee of getting paid for putting an entry into the contest) to win $20 (£10) for an hours work is measly. Say the competition gets 10 entries, that's £1/entry the contest holder is paying, ludicrous really. I'd definitely back putting a minimum of $75 on contests, however currently I am entering contests lower then that as I'm just getting into logo design and could do with the practice.