He has a jet because, honestly, he is traveling constantly for his work. Call it a tax on running multiple large operations. Given his long work days I can see how the jet pays for itself many times over.
And renting a yacht for $7k is actually super cheap. I know people who spend more on champagne in a club in a single evening, and no, they are not billionaires.
Even if this yacht was rented 365 days a year, which I do not believe, this would be $2.5M / year.
Even if this absolutely worst case (I don't believe for a second that Elon is partying on the yacht for large part of the year) you could still rent it for literally hundreds of years and still not match the cost of just buying your own large yacht. Forget about insurance, maintenance and staffing costs.
If you want to pass judgment on what is and what is not frugality when it comes to billionaires, you need to expand your views a bit and stop thinking like a poor person.