This is also an interesting article (book summary) about prices Ben, created by yc.user davidw I think: http://www.squeezedbooks.com/book/show/7/information-rules-a-strategic-guide-to-...
The economists' answer is doubtless, "what the market will bear", which is of course of no practical use. Thanks to the web, you could do some trial and error - some users get different prices, or you can run promotions, stuff like that, so you can see how people respond.
http://www . profesia . sk/en/employment/services-and-pricelist/client_index.php?action=services
Sorry, I don't want link to them