Well thats a million dollar question, i suppose it depends what you are offering in return. the best site i know that are making money is one that offers things like forum plugins and templates, you subscribe to the site, costs 5 bucks a year, and for that you can download everything they have to offer. Doesn't sound like a lot, but he has something like 6000 members so it soon ads up. So i think the best way to go is to offer something special for subscribers, and dont charge too much.