We're obviously biased...so we won't say "us."
Keri's message is great, but I would recommend doing the following when you're in the process of evaluating them:
-check out their title tag of their own website; it always confuses me that an SEO company doesn't SEO their own website. If you need help with evaluating other factors that might point out what they're trying to optimize for their own efforts, let us know (i.e., the h1, h2 tags, meta description, image file names, alt text, etc.)
-if you need reference on pricing, here's a good recent post from Rand about costs: http://www.seomoz.org/blog/seo-pricing-costs-of-services and another post debating/discussing the topic: http://www.seomoz.org/q/2011-sseo-pricing-costs-what-should-you-charge-how-much-should-you-pay
-definitely ask for more than 3 or so case studies, if possible.
-usually a quick answer is a wrong answer in our minds. Almost every client has different needs. We always hesitate when someone asks us "how much will it cost?" or "how long will it take?" While we have standard pricing created because many people do ask for the pricing details right away, I would argue that each case is like taking on a new patient for a doctor. There's a bit of history with that patient before they see the new doctor.
-thus, they should be asking you guys a lot of questions. Wouldn't you want a doctor to do that?
DigiJoltMan is right for the most part, but there are also a number of firms that charge a bit more than they potentially should. So, the reverse is true as well.
Gotta run, but let us know if you need any other questions. I think it's great you're asking here. Good luck!