One of the companies proposing this idea has created a common trading metric – a way to compare resources from multiple providers in an apples-to-apple fashion to ensure one commodity is equal to another. It's called the Workload Allocation Cube, or WAC from 6Fusion and it takes into account CPU, storage and input/output speeds to create a common metric that is applicable across multiple providers.
Some cloud providers have unique services making apples to apples comparisons difficult, but that doesn't prohibit a unique service from being sold on an exchange market as a futures commodity. The WAC, or some similar metric, could be used only on like products from companies.