What do you mean by this? Are marking you up or actually rerouting your orders and faking where the execution happened?
No, prop firms will always ( as far as I know ) add a haircut to the commissions they are charged from the FCM (s) they use. So, for example, if you traded 60K RT's per month and the firm collected a 5 cent haircut that would be $3K in revenue for that month. Plus, they want a desk fee. If there's a way that a prop firm can figure out how to take money from you - they certainly will.