The biggest money should be spent not on research, but on lobbing, by the way.

One of the ways to do it is to change regulation which will allow to create "human brainless clones" and a technology to transfer a brain from old body to the new one. (Now both technologies are banned, and head transplants banned even on rats).

Head transplant technology is now in its infancy but large investment could make it cheap and safe.

But other large scale investments in antiaging, cryonics, artificial organs digital immortality would also help.

Creation of safe AI in the next 20 years will also solve the problem.

I guestimate 1 trillion dollars a year for all it or even less.