Even as a non-profit, that isn't nearly enough. At that point, find somebody who believes in whatever cause you're supporting to donate their time. Offering $50 to a band is an insult.

Edit: just saw "20 minute sets." They'll spend more time setting up than they will playing at that rate. Also, is a PA provided? Sound guy being paid?

My advice? Get one act, pay them everything you were going to pay the multiple acts, far less headache and hurt feelings.