Happy to answer any questions about the project in here. It's all Open Source and parked on Github: https://github.com/stackmonkey. Thanks for taking a look!
The pre-pay model works well for reservations of a VM, but what are your thoughts about consumables that vary, eg Bandwidth? Would you imagine service providers just say, hey X VM includes Y gig of transfer per hour?
The appliance currently allows you to set an ask price for the instance size, but later versions could easily let you configure how much you charge for what resource, even doing so based on current demand on your systems.