Most people don't currently donate all their disposable income to charity.

I do. I give away all my earnings and my husband gives about 20% of his, so we live on a much smaller budget than most people we know.

While we live on a much smaller budget than many people, we still have disposable income that we could choose to spend on cryonics instead of other things. If cryonics cost $500/year you would still have $28/week in discretionary money after the cryonics spending. Whether this makes sense depends on whether you think that you would get more happiness out of cryonics or that $10/week. As for me, I need to read more about cryonics.

(Some background: As she wrote, julia is very unwilling to spend money on herself that could instead be going to helping other people. Because this leads to making yourself miserable, I decided to put $38/week into an account as a conditional gift, where the condition is that it can be spent on herself (or on gifts for people she knows personally) but not given away. So cryonics would not in our case actually mean less money given to charity.)