Design Studio Kickback?

[AndrewSA]

I get most of my commercial work through a design studio. I'm the first and probably only photographer who they go to when a client needs images for whatever project they are working on.

They have just been marking up my quotes to their clients as they would for any other vendor. Is there some protocol that I should be following for this?

Specify a cap for how much they can mark up?
Give them a kickback/finder's fee for all the work I get from them?

I'm thinking about discussing with them the possibility of them actually selling my services to their client rather than if the clients ask if they know a photographer, etc. What is a fair trade-off for something like this? They get a certain percentage?