There's a practical limit on how much they can spend on their lifestyle, so there's a great demand for status symbols. We can see this in the art market.
The people benefiting from this art bubble are mostly the slightly-less-rich who bought stuff when it was cheaper. How can we get in on this action? Well, taxing it is difficult when the art market is global. Selling off the contents of the National Gallery is probably a non-starter, and in any case we can only do it once - it won't provide an ongoing source of income.
Anyhow, art isn't the thing, it's just a good demonstration of the willingness of high net-worth individuals to pay silly amounts of money for status symbols. See also: football clubs and newspapers. Perhaps we should create a status symbol of our own to sell to them. Something that can be made by the state for zero cost, and can't be replicated by any other actor. And something that doesn't represent an actual difference in material conditions.
I have just the idea - peerages! Once the House of Lords reform goes through, the peerage will be disconnected from the legislature entirely. What better way to get the nouveau riche to voluntarily pony up some money for society than to sell them prestigious titles? We already know people are willing to pay for honours - there might even be more people interested if it weren't so shady.
The system I propose is quite simple. Every year, set a blind auction, with a number of life baronies available for sale, along with one viscountcy. We'll see how much money we can raise. We run the risk of peerage inflation, so we'll keep earldoms, marquessates and dukedoms in reserve. I have absolutely no idea how much money we'll raise - the market will decide that, but in the worst case where it doesn't cover its administrative costs, we can just discontinue it after a couple of years.
Why not? I mean, what's in danger - the dignity of the honours system?