I want to create a registration form for monthly dinners at non-profit senior center. How much does jotform cost?

I have a paypal account and expect 100 registrations/payments per month.

A free account allows processing up to 3 payment submissions. If you need to receive up to 100 payments you may upgrade to Silver. However, if you need need to receive more than 100 payments, then a Gold plan would be better: https://www.jotform.com/help/331-How-to-Upgrade-My-Account 

Follow this guide to integrate with PayPal:

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form

Let us know if you need more help.

You may visit our pricing page: https://www.jotform.com/pricing 

If your organization is non-profit, you may apply first to the non-profit discount: https://www.jotform.com/nonprofit/pricing/