As to your question, I'm facing the same problem. I've got a couple of hundred free users who signed up to help beta testing, I think I'll allow them to keep their free accounts as a thank you. Beyond that, I'm going start high and see how it goes. The system is designed to allow certain features to be enabled/disabled easily so I'll offer 3 different price options.
I think it is about finding that sweet spot on perceived value. I'd prefer to have the product priced slightly on the higher side with fewer customers (but roughly the same net revenue), that way I'm just as well off but with a lower support requirement. Plus it gives me room to price-cut in the future.
And then there is always to option to try and split test pricing to see how sensitive your prospective customers are to price adjustments.
Above all, I'd look at other successful companies like Apple and see how they try and structure their products and prices in such a way to convince you that you're actually getting a good deal.
Oh and of course, try and calculate what it costs you to provide the product and support per signed up customer... and don't charge less than that ;)