Today, we do not know what the market data costs are, or whether they represent the true or actual costs of providing market data. Using an estimate is even more nebulous than what is done today. The release had revenue numbers, but nothing for expenses. It would help to have real expenses for a determination of what is reasonable. The only way to ensure fairness and reasonableness is to use actual costs.
Indirect cost are subject to too many interpretations.
Even using CPA's, you would never get a consensus of what cost would be appropriate to include in market data expenses. You only need direct costs. Otherwise, you will never know what is included in the total base. The number of executions should be used as the primary basis for compensation, and not quote traffic. Quotes only reflect traffic in the network.