I offer it now for free. I don't feel like I'll ever get to the point of offering paid sponsorship..but maybe in a few years.
When I paid for ads, I look for blogs that blog consistently, have a large following (blog follow, facebook, twitter, etc) and get a lot of feedback on their posts. They also need to have a well designed website/blog. I also take into account what I get for what price and of course if they blog to my target audience.