Leave that for later. Ask other nonprofits that have developed Web sites to give you positive referrals. You can also surf the Web and find nonprofits whose Web sites you like and try to track down the developer. It's often listed on the site, or you can call the organization and ask. People are usually happy to tell you one way or another if they liked the firm they worked with.
Firms tend to specialize in working with specific sizes of organizations.
You're often better off with a firm that's worked with organizations similar to yours. Finally, ask for references and call to check up on them. Regarding cost, don't put this in the RFP, but you can discuss your range when you make contact with a prospective bidder. Once you get a bid, don't hesitate to negotiate on cost. The answer will help you evaluate the pros and cons of the firms.