Until then, I'd be more interested in donating to general life extension research than paying for cryonics specifically.

This is very similar to my primary objection to cryonics.

I realize that, all factors considered, the expected utility you'd get from signing up for cryonics is extremely, if not infinitely, large. In any case, it's certainly large enough to be worth the price.

However, it seems to me that there are better alternatives. Sure, paying for cryonics increases your chances of an unlimited life by many orders of magnitude. On the other hand, funding longevity research makes it more likely that we will ever overcome aging and disease. Unlimited life for most or all of the future human population is far more important than unlimited life for yourself, right? (One might object that life extension research is already on its way to accomplishing this regardless of your contributions, which brings me to my next point.)

If an existential risk comes to pass, then no one will have a chance at an unlimited life. All of the time and money spent on cryonics will go to waste, and life extension research will have been (mostly) squandered. Preventing this sort of risk is therefore far more important than preserving any one person, even if that person is you. To make matters worse, there are multiple existential risks that have a significant chance of happening, so the need for extra attention and donations is much greater than the need for extra longevity research.

To summarize: Cryonics gives you alone a significant chance of gaining unlimited life. Working to prevent existential risk gives billions of people a slightly increased chance of the same.

It seems to me we shouldn't be spending money on freezing ourselves just in case a singularity (or equivalent scientific progress) happens. Instead, we should focus on increasing the chances that it will happen at all. To do anything else would be selfish.

Ok, time to take a step back and look at some reasons I might be wrong.

First, and perhaps most obviously, people are not inclined to donate all their money to any cause, no matter how important. I freely admit that I will probably donate only a small fraction of my earnings, despite the arguments I made in this post. Plus, it's possible (likely?) that people would be more inclined to spend money on cryonics than on existential risk reduction, because cryonics benefits them directly. If someone is going to spend money selfishly, I suppose cryonics is the most beneficial way to do so.

Second, there's a chance I misestimated the probabilities involved, and in fact your money would be best spent on cryonics. If the Cryonics Institute webpage is to be believed, the cheapest option costs $28,000, which is generally covered by insurance, costing you $120 per year (this option also requires a one-time payment of $1,250). Unfortunately, I have no idea how much $1,250 plus $120 per year would help if donated to SIAI or another such organization. Cryonics certainly give a huge expected reward, and I'm just guessing at the expected reward for donating.