Most of what people buy fills their marginal desire. The next most desired good. For instance, perhaps you are in the desert, your first most desired good may be water (discounting rescue), next most would be shade, third most would be food, forthmost would be transportation. You would go for the highest desired first then work down your list. It’s the cost of the last one you are willing to by that determines your marginal cost.

Rich people have a higher priced marginal cost than the rest of us and can go further down their list for goods they desire.