Without getting into specific numbers surrounding this kind of price model, the basic idea is that for XYZ program you pay A rate per B time period. While charging per hour is probably not the best idea, charging per minute or even second could be quite feasible. So how much do you charge for each unit of time? Well, obviously it's going to vary depending on the cost of the "retail" version of the software. The important thing is the concept: you only pay for the software when you're using it; when it's sitting idle it isn't costing you a dime.As always, Geoff raises some interesting (and most likely controversial) points, but he makes a convincing argument to back up his idea.