What are the social conventions for being asked to donate money at work?

I don’t have a problem saying no, necessarily, and it’s to a good cause (students in financial distress). BUT - and it’s a significant but - with the cut to my teaching load last week, I am also in financial distress. (There was a sensitivity training for students in financial distress the day that I found out I was back to poverty wages. Sitting through it while not screaming and as my colleagues ooh’d and aaah’d about the unimaginable stresses of the kids was an exercise in character building.)

The coworkers know my deal, theoretically, and i’m concerned if I say it too many more times/ways, I’m going to forever be That Bitching Adjunct, but they won’t stop asking. (“Really, any amount will help - even if it’s only $20,” is a response that comes to mind. The husband has forbidden me from screaming, “JUST TWENTY DOLLARS IS TWO AND A HALF HOURS AT THE GAP.”)

I don’t necessarily need a real solution, but snappy one-liners are appreciated.