Really the first thing you must decide is that you need to charge as this is the only real way to get validation that what you provide is valuable. Someone needs to give you real dollars for what you do.

As for the actual price there are a number of ways to attack the problem but the key is the price will change over time as you learn from the marketplace and your customers.

- Ask your most active and least active users what they would be willing to pay and test them after they respond by asking them to actually pay what they suggest

- Survey the market and decide if you want to be low cost, high cost, or best value. This will give you a range of where to start

- Work with a pricing expert like http://sixteenventures.com