What kind of virtual lab are you wanting to run? Its unlikely that you'll find a pay per session option available. Think about it, to pay by session, you would have to buy the service, set up your lab, run your simulation, and then the company would have to remove your entire session to make room for other customers. That would take more than a couple of days, and just wouldn't be profitable for the company.
Being a little more specific might help, but I definitely can't see this working outside of a monthly access sort of thing. It's the only way the company could make any money off of it