In Marginalism, you compare utility from agent to agent but you can never attribute an absolute value.
how do you decide how to allocate it between rent, food, savings, entertainment, etc.
most rational people would use some type of internal or subconscious utility calculation to decide what how to budget their income
i dont know what marginalism is or why its useful when you can simply approximate a value by looking at the price of the good/service and use that to compare with other goods and services
Money is a reasonable approximation of "aggregate utility" most of the time, but that holds only so long as interactions between individuals remain voluntary and there is enough trade in similar items to establish a "customary price". It doesn't work well for goods which are not customarily traded for money, or as a basis for orchestrating involuntary interference in the market.