I would donate $10 per month to a decent open source Mastodon client, at least while I was using it regularly (and would have done so for Tweetbot).
I donate to a number of open source projects, a decent number of them regularly. I am lucky enough to be able to afford to buy software when I need to and donate to projects I think are worthy of support. It makes me extremely happy every time I am able to donate to an open source project instead of buying software. It makes me sick every time I capitulate and end up renting softare. Software subscriptions can die in a fire.
I'd encourage you to donate if you can afford to, but if you can't, that's ok.
A world dominated by open computing platforms and software is such an exciting prospect that it's worth putting a bit of money and effort into. And if that doesn't pan out, at least you helped the maintainers a little!