I would, generally, avoid any short-term developer who's charging under $50/hour.
The other thing to note about elance, vworker, etc. is that there are a number of companies in very low-cost places (such as India, Pakistan, etc.) who will then re-distribute the work to their own employees. They tend to produce very low quality work, and there have been some horror stories about these types of developers using the job to insert nastiness into your site for their own ends (why hack a site to insert your malware when someone will give you access and pay you for it?)
YMMV. Treat them like any other person you want to employ and check them out before giving them much access or responsibility.
I.e. I do front-end and back end development and my rate its around $45 per hour. Its probably on the cheap side, but I am trying to extend my customer network.
In Spain, which is western Europe, but cheaper than other countries like UK, Germany or France, hourly rate for a freelancer can range from 15€ to 60€, being between 25€ and 35€ the most common pricing.