i'm donating 5$/mo to no less than five open source patreon/opencollective right now
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!
Linux Weekly News ended up with this kind of model. A long time ago back in 2002, LWN was about to shut down, but those of us in the Linux community found it had tremendous value and asked for options to pay, as well as options to pay more. Here we are 2 decades later and LWN is still around! Sometimes it pays to let your users help out financially.
Tweetbot apparently cost $6/year, so are you saying you would donate $60/year if they released an open source Mastodon client? Are there other open source apps you donate to?
I'm genuinely curious how people approach payment/donations for open source software, I'm not trying to pull some gotcha on you. I don't donate to any open source projects, but I feel that I probably should.