That was the biggest detractor when I started to try some of these sites out during my latest stint of being laid off. There would be lots of requests for "clone eBay/etc" with 15 bids from outsourced developers offering to do it for < $200.
The signal-to-noise is often too low on these sites.
Even then it's fairly challenging to modify some crappy script for $100. My experience with elance was that development was just a no-go, but you could make some money with other areas, for me system administration. You could get $100 to modify some script, or the same $100 to _install_ it.
A few years ago I hired a guy from the Ukraine, he only wanted 2$/hour. I couldn't believe it and paid him more, but at that rate you have 50 hours to customise. Not too hard.
The signal-to-noise is often too low on these sites.