Membership sign-up system

Hello, we are a small non profit organization looking for a software development company. We have already designed the site and want to add some features to it. We want a membership sign-up system where people can pay online and be issued a passwordto access member-restricted portions of the site. We do not have a very big budget we need someone who is cost-conscious, but who has the ability to set up these features. Any advice on searching or on specific companies would be much appreciated.