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.
Price improvement from all markets will drive the executions. All exchanges, third markets, and ECN's should be linked together for transparency and price discovery. Routing orders to a specific market does not mean the order will be executed, since someone else may get there first, or the size is not available in that market. The option exchanges should also be linked, so customers do not see a trade through. Quote expenses for professional end users could be billed through the quote vendors without a markup.