One friend of mine is a developer at a nonprofit, and I know he's earning in the 25th percentile for his skills and experience in this region. I am certain his passion for the cause helped lead to this. Ironically, this situation has, over time, eroded his passion for the cause!
Nonprofits lose a lot of good people doing this, but they don't seem to care. Maybe because they've always got fresh meat ready to take someone's place? Maybe because donors always pressure for low overhead costs? Hard to say, but it's foolish. Only harms everyone involved.