High level, there are two ways you can go with this: Subscription model or payment for individual transactions/access to content. Subscription sounds like the best model here.
Judging from what you write, membership is a new thing for your site. So you should be free to price it as you want to. But remember, that whatever price level you are currently at, will serve as an anchor for your existing customers.
Also know, that no matter how much more you bring to the table, if you are too expensive, your current customers will look for alternatives. But it could be, that you are really better served by having a smaller volume of customers, if they just pay more.
My first step would be to understand on an individual basis, who your customers are, how they use your existing service, and how you think they will use your future service. Do phone interviews, surveys or something like that. If you are alone with this service, your customers should be loyal, and willing to offer their opinion to you. After that, you can decide on a viable business model.
Feel free to contact me, if you would like to discuss this further. I offer first call for free.
Good luck with the expansion!
Best regards
Kenneth Wolstrup
