I am a developer and own a web/mobile development agency. I had some success with startup as the technical co-founder. I get pitched few times a week by people saying they have a great startup idea, and if they develop it they will give me some equity. How do I tell them if you cannot manage few thousand dollars for developing a product they shouldn't be entrepreneuring and go get a real job - without sounding like a jerk. I am okay if people ask me if I can develop it for a lower fee and some equity. I really get agitated when people ask for free work. How do I convice them they should pay. Most people who ask for free work can afford to pay.
You "convince them they should pay" by publishing your 'no equity-only' policy on your agency's website!
I did a version of this several years ago, when I was the head of operations for a digital agency. Sales leads kept saying they wanted to "partner" on mobile apps (that is, no cash up front). I updated our website to reflect a minimum price for app development. Overnight, our sales prospects were much better-qualified.
From what you've described, try something like this: "We provide development services for clients who budget at least $X for a prototype. We consider accepting a mix of cash and equity for 1-2 projects each year. We receive dozens of proposals each month; if you can't afford 100% cash, please indicate the terms you have in mind when you complete our client application."
Want help customizing a solution for your agency's situation? I'm glad to do a call to help. Good luck!