Buying real Estate is a good investment. Even better (when you have enough money) if you or a member of your family lives in that house. So does it matter if Charles bought it or not?
"The British taxpayer" should stop being judgmentally being interested in Harry & Meghan. They don't pay for them anymore, so H&M are now just private citizens the media has an unholy interest in. Charles may give them a part of Harry's inheritance already, so what - he has private money after all.
I did not read the book about them but learned from rather anti-monarchistic media that Harry had indeed reason to protect his wife from the Royal environment and the British tabloids with their claims about "the taxpayer" this and "the taxpayer" that.
And honestly, Harry was the spare of a new generation and, with William's children, is no longer needed as part of the working force of the RF. As lovely as that would be for me as a Royal watcher! But they did not mean a lot in the hierarchy of the RF as they were shown by the Royal establishment and so they resigned. They even would have been willing to work occasionally, but that was considered to be "an honour" for them and not wanted when they would live like private citizens.
I'm just sorry there was not enough patience in them to get a better deal, but considering the media and the people in Royal service who were stirring negative emotions against them, it was maybe too difficult for them. I find that I still can understand them, not as Royals, but as human beings and I hope they find what they are looking for in California and maybe can be convinced to work a bit for the RF later on.
As for little Archie: the climate in California is better than in London, he now lives in a gated community with other kids in a similar situation. Not like a poor princeling, but the son of two well-off public personas. More like his distant cousins of the former Greek Royal house who as well have an American for a mother and were raised there for a time. If I could choose for a future grandchild to grow upo like Prince George or Master Archie, I'd choose Archie's current life as well.