when you're talking about a host, you hopefully mean a Virtual Machine, otherwise I would recommend to bring them back to a baseline.
Apart from that (let's say misunderstanding) I can recommend VMware Professional Services to calculate the cost factor of resource run-times including to facility; power, cooling, maintenance, hw/sw etc.
If you don't resell your resources but need it just for your own reference, keep it simple and use the fix mode.
I'm sorry that I can't provide a golden formula how to calculate the costs of a Virtual Machine. It actual doesn't exist.