With costs being covered, there is no reason for rebates. Rebates are like payment for order flow, someone is paying for the excess funds. The only way a rebate would make sense would be for a percentage rebate to all users at the end of a year based on total payments, if their were excess revenues available from the Plan's and exchanges. The size of firm should not be a factor in setting a minimum. The quote vendors have the same administrative cost for a one-page invoice or 100 page invoices.
Vendors already send bills to users', use the current system to the best advantage.
With the for-profit models being considered by many trading locations, the trading entity and the SEC could pay for a separate division for market regulation. The regulatory division of a trading location should bill the trading portion of the entity for market regulation and support. The Plans could use the quote vendors to collect the fees. Some end users currently have to pay deposits with quote vendors, and that deposit amount may need adjusting by the quote vendors. The Plans could capture their cost first, and then the exchanges and trading locations.