Most of those sites are pretty tough for the smaller independent freelancer, especially Elance, RentACoder, or Guru. The reason being that they are dominated by low cost programming conglomerates in countries like India, Russia, etc.
So, whatever you think you can charge for your work, you're guaranteed to have someone charging 5 bucks or less for the same work due to their massive resources (and thus their ability to work dirt cheap).
I'm sure you can make a go at it, but there is definitely substantial competition.
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.
So, whatever you think you can charge for your work, you're guaranteed to have someone charging 5 bucks or less for the same work due to their massive resources (and thus their ability to work dirt cheap).
I'm sure you can make a go at it, but there is definitely substantial competition.