True it does avoid transmission fees but not distribution fees. But that is irrelevant as Transmission is a pass through cost. Anyway, surely this just means you should be calculating based on the average spot rate at the closest GXP? Complicated area (so I probably have it wrong) - keep it simple - if you are buying power generation the same costs should apply whether it is from a large hydrodam, windfarm, gas generator or a small solar, wind turbine. That price is the spot wholesale rate.
There is also additional metering costs (a second meter is needed - it doesn't run backwards), admin costs (an invoice must be created and posted/emailed), and profit (nothing is free).