This guy is only worth $3B, he may be spending half of his entire net worth to make this happen. I hope he inspires the richest billionaires of the world to fund such lofty ventures.
$10M-$20M is more than sufficient for a few generations (in a 100-200 years, our current understanding of the concept of money might not even be relevant, so it's pointless to plan for anything more than that). Anything more is just throwaway cash IMO. If you had the opportunity to become the first private citizen to go to the moon, you'd be stupid to not cement your legacy like that.
He hinted during QA it was 5% of BFR development costs, and Elon said a "material" percentage, which for expense reporting usually means >= 5%. Dev costs were said to be $5billion, so that would be $250,000,000.
The questioner asked "is it more or less than 5%" -- I don't think they were confirming 5%, just repeating the number back (and Musk refused to say anything about the percentage at all).
Besides the fact that he isn't spending $1.5B on it, really, what can you do with $3B that you can't do with $1.5B? There's really not that many more things that you can do. So spending it on a (hopefully not) once in a lifetime opportunity doesn't sound like that bad of an idea.