I just released http://www.examcommunity.com about two weeks ago... its a ghost town now... hopefully that will change.. How long did it take for you guys to get some traction?
We initially geared our site towards teachers, but later learned that wasn't the market that was willing to pay for our services. Maybe your experience will be different.
It is a slow process. About 1-2% of our customers convert to paying accounts. When we started limiting what was free it seems people stepped up to the bat and subscribed to a paying account.
Note - your prices seem very low. You could experiment with your price structure, people might value your service more than you realize.