My naive linear model is that ~$400 billion research funding currently spent per year buys about 1 year increased lifespan per decade, so it would take about $4 trillion per year spent on research to stop aging, or a one-time investment of $80 trillion. For 99% confidence I'll add a safety factor of 4, yielding a one-time payment of $320 trillion, or $16 trillion per year. In other words, this back-of-the-envelope guess suggests the entire economic output of the United States would be just sufficient to discover and maintain an aging cure.
I don't think that all of the increased lifespan is due to healthcare spending. Various enviromental regulation likely increased lifespan. Our current paradigm of drug development unfortunately get's exponentially more expensive via Eroom's law. I doubt that simply spending more money in the same way produces linear progress.
I think the problem of the mythical man month also exists in research. You can't simply throw in more money. Given researchers money can often make them spend money on fancy equipment instead of thinking hard about what to do.
As far...
How much money would it take to engineer biological immortality for at least half of the world's population, within 20 years, with 99% confidence?