I think there is a tale to tell about the consumer surplus and it goes like this.
Alice loves widgets. She would pay $100 for a widget. She goes on line and finds Bob offering widgets for sale for $100. Err, that is not really what she had in mind. She imagined paying $30 for a widget, and feeling $70 better off as a consequence. She emails Bob: How about $90?
Bob feels like giving up altogether. It takes him ten hours to hand craft a widget and the minimum wage where he lives is $10 an hour. He was offering widgets for $150. $100 is the absolute minimum. Bob replies: No.
While Alice is deciding whether to pay $100 for a widget that is only worth $100 to her, Carol puts the finishing touches to her widget making machine. At the press of a button Carol can produce a widget for only $10. She activates her website, offering widgets for $40. Alice orders one at once.
How would Eve the economist like to analyse this? She would like to identify a consumer surplus of 100 - 40 = 60 dollars, and a producer surplus of 40 - 10 = 30 dollars, for a total gain from trade of 60 + 30 = 90 dollars. But before she can do this she has to telephone Alice and Carol and find out the secret numbers, $100 and $10. Only the market price of $40 is overt.
Alice thinks Eve is spying for Carol. If Carol learns that Alice is willing to pay $100, she will up the price to $80. So Alice bullshits Eve: Yeh, I'm regretting my purchase, I've rushed to buy a widget, but what's it worth really? $35. I've over paid.
Carol thinks Eve is spying for Alice. If Alice learns that they only cost $10 to make, then she will bargain Carol down to $20. Carol bullshits Eve: Currently they cost me $45 to make, but if I can grow volumes I'll get a bulk discount on raw materials and I hope to be making them for $35 and be in profit by 2016.
Eve realises that she isn't going to be able to get the numbers she needs, so she values the trade at its market price and declares GDP to be $40. It is what economist do. It is the desperate expedient to which the opacity of business has reduced them.
Now for the twist in the tale. Carol presses the button on her widget making machine, which catches fire and is destroyed. Carol gives up widget making. Alice buys from Bob for $100. Neither is happy with the deal; the total of consumer surplus and producer surplus is zero. Alice is thinking that she would have been happier spending her $100 eating out. Bob is thinking that he would have had a nicer time earning his $100 waiting tables for 10 hours.
Eve revises her GDP estimate. She has committed herself to market prices, so it is up 150% at $100. Err, that is not what is supposed to happen. Vital machinery is lost in a fire, prices soar and goods are produced by tedious manual labour, the economy has gone to shit, producing no surplus instead of producing a $90 surplus. But Eve's figures make this look good.
I agree that there is a problem with the consumer surplus. It is too hard to discover. But the market price is actually irrelevant. Going with the number you can get, even though it doesn't relate to what you want to know is another kind of fake, in some ways worse.
Disclaimer: I'm not an economist. Corrections welcomed.
Bob is thinking that he would have had a nicer time earning his $100 waiting tables for 10 hours.
If that job's available, why doesn't he do it instead? If it's not, what's the point of focusing on his wishing - he might as well wish he were a millionaire.
The missing detail in your story is what Bob did to earn money while Carol's machine was working. If he was doing something better than hand-making widgets, he wouldn't go back to widgetry unless he could sell at a higher price. And if he was doing something less good than making widgets, he's happy tha...
If you believe that science is about describing things mathematically, you can fall into a strange sort of trap where you come up with some numerical quantity, discover interesting facts about it, use it to analyze real-world situations - but never actually get around to measuring it. I call such things "theoretical quantities" or "fake numbers", as opposed to "measurable quantities" or "true numbers".
An example of a "true number" is mass. We can measure the mass of a person or a car, and we use these values in engineering all the time. An example of a "fake number" is utility. I've never seen a concrete utility value used anywhere, though I always hear about nice mathematical laws that it must obey.
The difference is not just about units of measurement. In economics you can see fake numbers happily coexisting with true numbers using the same units. Price is a true number measured in dollars, and you see concrete values and graphs everywhere. "Consumer surplus" is also measured in dollars, but good luck calculating the consumer surplus of a single cheeseburger, never mind drawing a graph of aggregate consumer surplus for the US! If you ask five economists to calculate it, you'll get five different indirect estimates, and it's not obvious that there's a true number to be measured in the first place.
Another example of a fake number is "complexity" or "maintainability" in software engineering. Sure, people have proposed different methods of measuring it. But if they were measuring a true number, I'd expect them to agree to the 3rd decimal place, which they don't :-) The existence of multiple measuring methods that give the same result is one of the differences between a true number and a fake one. Another sign is what happens when two of these methods disagree: do people say that they're both equally valid, or do they insist that one must be wrong and try to find the error?
It's certainly possible to improve something without measuring it. You can learn to play the piano pretty well without quantifying your progress. But we should probably try harder to find measurable components of "intelligence", "rationality", "productivity" and other such things, because we'd be better at improving them if we had true numbers in our hands.