The point of the quote is to demonstrate that using poor-folk calculations to try and figure out what things cost for rich folk is pointless, and has been for the past ~100 years.
The rich man’s mentality can also work for middle class people if every purchase they make is a long-term well-informed decision, rather than flying with the pop culture of what everyone else is currently buying, wearing or visiting. For example, buy timeless durable unfashionable clothing, buy a used but durable car from which you can extract the best value in the long run, buy products which can be utilised for many purposes, etc.
With enough money, you can afford to wait for a good deal because you don't need the money right now to live. When you're wealthy, you can brave health issues, accidents, sudden crossings with the law, etc. without ever truly touching the money-in-wait for a good deal. When you're middle-class you definitely don't have as much cushion.
This is similar to the logic of "the best time to buy is a recession", except its difficult to buy during a recession purely because it's more likely that a middle class person's cushion is gone due to economic conditions and thus has no money to buy good deals.
Take boots, for example. He earned thirty-eight dollars a month plus allowances. A really good pair of leather boots cost fifty dollars. But an affordable pair of boots, which were sort of OK for a season or two and then leaked like hell when the cardboard gave out, cost about ten dollars. Those were the kind of boots Vimes always bought, and wore until the soles were so thin that he could tell where he was in Ankh-Morpork on a foggy night by the feel of the cobbles.
But the thing was that good boots lasted for years and years. A man who could afford fifty dollars had a pair of boots that'd still be keeping his feet dry in ten years' time, while the poor man who could only afford cheap boots would have spent a hundred dollars on boots in the same time and would still have wet feet.
This was the Captain Samuel Vimes 'Boots' theory of socioeconomic unfairness.” ― Terry Pratchett, Men at Arms