How do I figure out what to charge for my hosted services?

I'm looking for cost and pricing equation formula templates for offering hosted services - and I can't seem to find anything.

Anyone have any good sources for formula for figuring out the costs of hosting clients and how to reasonably price those services for your customers? How do hosting companies figure out that they're going to charge $150/month for a 20GB VM, or $7.50/mo. per user for a hosted Exchange server?