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.