We'll charge the same as the cloud provider charges us, plus a small percentage to cover the overhead of storing game server config, running the admin console, etc.
From initial research we estimate charging a price of $0.05 per hour for a 20-player Minecraft or standard KF2 server. This works out to be much more than the pay-per-month equivalent, but we only expect usage for short bursts like on some nights or over weekends which should leave users with bills well under our competitors'.
Would you use such a service? Would you prefer it to the normal pay-per-month providers out there? What criticisms do you have of the setup?