Choosing the right SEO firm is more than a matter of price. It's important to choose an SEO firm that is qualified in all aspects of search engine optimization: keyword research, content development, web design and coding and off-site promotion.
Check the PR of their own site. If it isn't at least 4, don't bother with them. Also find out where they get their backlinks from. If it's from a "farm", again, don't bother. Angies list is the most reputable one FYI.