The fair price to pay for your web site is determined by which designer/firm seems best equipped to build the site you envision for your business. Ask for web design samples -- you'll see differences in styles that will tell you whether the designer is foremost a programmer or whether they are a skilled graphic designer.
In addition to look and feel, ask them whether they write the copy for their clients or are provided the content for pages. Usually, a much lower hourly price reflects a business that is more one-dimensional -- you're getting a nice site design, but no expertise in online copywriting, SEO, site planning, etc. The higher end firms will have specialists in all those areas, and a specialist in each area to work on your site.
If you're looking to your site to generate traffic and leads, bite the bullet and use a company that writes SEO-optimized content along with designing the site. We've had lots of clients that came to us for their redesign after going to less-expensive designers – they ended up paying twice to get one good site – not a very cost-effective solution.