Are you assuming multiple suppliers for that cure or a single supplier?
Very different. In the case of a single supplier - wouldn't it make sense to pay everything you own ( minus what else you need to live ) to survive?
In the case where there were multiple suppliers ( with no collusion between them ) - then the price could be driven down to the cost of production, rather than the value that it brings.
So for example food is essential to live - yet it's cheap.
That's assuming you get to negotiate with every customer. If it's one price for everyone then while what you say is true for each person's max (ignoring the complexity of people looking after others, the same logic applies of "everything you can afford") that's different for everyone so the optimal price can vary.
The solution there is to sell the same thing to different people at differences prices - lots of ways of doing this.
For example America pays lots more for it's pharma drugs than other countries.
Assuming free market, the cost of total cure will probably settle on at least 2-5x of an average life expectation times a treatment cost per year, minus some minor operational expenses.