One of the easiest ways to gauge the low-market value for design work is to check Elance.com projects
This tends to reflect the floor of web project (in US dollars) as eBay would tell you the fair market value for a given product.
Be forewarned, there's a lot of underbidding going on with Elance -- but it's a pretty good snapshot of what your project would be worth to other people with a need for it.