What kind of therapy?
Assuming you’re talking about money for the child’s counseling and not speech or physical therapy:
“about how much do you think would be appropriate to contribute monthly?”
I say it would be inappropriate to contribute anything at all. His life didn’t even start yet, and already you’re looking for solutions to non-existent problems? “An ounce of prevention is worth a pound of cure”. If you have your own issues, spend the money on therapy for yourself so that your kid needn’t suffer. Or on a parenting workshop. Build your kid’s self esteem, be alert to and aware of issues as they arise, love your kid and tell him about it, give him security- and you’ve saved yourself lots of money and heartache down the line.