Pricing models are tough. Unfortunately, we are programmers / engineers, not marketing people (Mike has a marketing degree but we do not let him do any of that).
When we started we did not have a clue about how many people were even going to use the service. Also, we thought the inconvenience of only having a 5 minute session for free, plus having to wait in line, would be enough 'encouragement' for professionals to opt for the premium service.
Boy were we wrong.