Never Work For Free!
Harlan Ellison explains it best; Never work for free! (some NSFW language), not even for non-profits, they are non-profits but they don't get electricity for free, their president doesn't work for free, none of their employees work for free, the cleaning staff don't work for free, etc.
Non-profit means they aren't in business to make a profit, but they do make money and they do have money to spend, how else would they be non-profit if they didn't spend what they took in?
In the USA, you can't take off free work for a non-profit on taxes either. It is a lose-lose situation. You lose your peers lose, and you dilute the value of the market for your peers! Never work for free!
They will take advantage of you, you will eventually realized you have lost thousands of dollars and quit, and they will find some other sucker and use you as an example of so and so did all this for free, you should take it up and continue working on it for free. Like I said, everyone loses, even the non-profit; because they get what they paid for, no one will really put any priority on something they are working on for free.
There is extensive discussion about this on programmers.stackexchange.com and really old questions ( that are off topic now that Programmers exists ) on Stack Overflow. Look there you will see the same answers by everyone.
Not working for free doesn't exclude you from not making a profit, it just excludes you from not charging something ( at least going rates ) to at least to break even.
But I think you already have decided on this "... because it wouldn't be worth my time ..." is in your question.