Sam Vimes Theory of Economic Injustice
From Discworld & Terry Pratchett Wiki
The Sam Vimes "Boots" Theory of Economic Injustice runs thus:
At the time of Men at Arms, Samuel Vimes earned thirty-eight dollars a month as a Captain of the Watch, plus allowances. A really good pair of leather boots, the sort that would last years and years, cost fifty dollars. This was beyond his pocket and the most he could hope for was an affordable pair of boots costing ten dollars, which might with luck last a year or so before he would need to resort to makeshift cardboard insoles so as to prolong the moment of shelling out another ten dollars.
Therefore over a period of ten years, he might have paid out a hundred dollars on boots, twice as much as the man who could afford fifty dollars up front ten years before. And he would still have wet feet.
Without any special rancour, Vimes stretched this theory to explain why Sybil Ramkin lived twice as comfortably as he did by spending about half as much every month.