The problem is that I value my first gallon of water every day very highly for drinking. The next gallon also pretty highly for drinking and brushing my teeth. The next two gallons are for cooking and stuff, so they're less precious; the next five gallons for bathing and sanitation; the next 10 for cleaning and washing, etc. All the way down until we get to the market price which in Houston for water that comes out of the tap is $0.01 per gallon.
I think it would be utterly exhausting to list out every subjective value that I place on every thing that I exchange for every transaction of every day. It's something that I can do in my head with very little effort since for many transactions the value/cost delta is high and I don't need to do careful accounting.
One of the great things about money is that it has basically zero transaction cost as compared to bartering which has a substantial either time cost to find the best price, or price cost to find a deal in a short period of time. Engaging in these kinds of nonlinear transactions (if they need to be done honestly) would bring an economy to its knees.