$0.
I breed five or so species of cockroach, so I never have to buy feeders. However, I do wind up having to feed them quite a bit of healthy vegetable matter to keep them well-fed and thus ensure the health of my collection.
To breed roaches expect a one-time payment of around $20 or so, then plan to not use them for a few months. It takes a while for a breeding colony to get to a size large enough where you can routinely feed out of it without risking depleting the population to the point where it will crash.
You can also spend more to get a larger colony that's ready to start feeding off immediately.