When to raise prices?

I'm running a media business where our business model is similar to that of the UK newspaper The Guardian, which is that the main content is free and we're supported by monthly contributions.

Right now we have 3250 supporters who contribute with either around $6/month or $55/year.

For this they get access to a private facebook group, online events, discount in our web shop etc.

I know that some people value what we do so much that they've signed up for more than one contribution plan, and some have even asked us to raise the prices.

How would you say that we know when we could or should raise prices?