It doesn't matter what its based on, just tell me what you charge.
if you charge $10/trade for the first 1000 trades, and $8 for 1001-5000 trades, and $5 for 5001 to 1 million trades a month, so be it.
You obviously have a charge table that you could give.
Based on this short conversation, I think you are a moron, so you can keep your charges to yourself.
good luck to you finding a customer.