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!