If it’s a painful process that requires I stay strapped to a machine forever, I’d pay nothing. If it’s a single pill with no harmful side-effects, you can have my firstborn if I ever change my mind about having children.
If you have the technology, you may be on your way to becoming the first trillionaire[0].
This was a curious question to come across, as I was just rereading about turritopsis dohrnii, the immortal jellyfish[1].
But as a first guess, about 10 to 15% of my net income.
From about $100k to maybe $300k. After that I might be able to afford more.
There's no doubt it could be an attractive product (assuming a bunch of stuff about how it works) but as a practical matter, living longer doesn't make me any wealthier, so it'd have to come out of my discretionary spending. While I might not have to save for my retirement due to age, I'd probably prefer not to have to work 50+ hours a week forever, so I'll probably need to save and invest such that I can get a sufficient income to enable me to follow my interests -- some will involve being paid, others might not.
Once I've acquired a house, or perhaps a couple, and have investments providing a post-inflationary, capital-preserving return that I can live off, then I'd likely look to move beyond a basic salaried employment into something more flexible.
But if I still need to pay for my "live forever" pills, there's a limit to what I'd be prepared to fork over on an ongoing basis.