If you're on such a low production budget that you just physically do not have the lamps to light a screen, then you really have to ask if green screen is the right option. Maybe flip it and shoot black limbo so you do not need lights, and the lights you do have can be better used as key lights for separation. You also don't have to worry about the color cast from your light screen. Essentially, you just need a garbage matte for the key, and then clean up what might be getting keyed that you don't actually need. Detecting foreground subject from background is so capable now that a screen isn't necessary, and matte clean up is pretty much unnecessary. Of course you lose street cred of not being able to say you used green screen, but who cares as long as the shot works out.

Sometimes the cost is human expertise. If the tech allows you to get stuff done with less competent staff, it's a win.