One thing I will always remember from my college economics class was the professor saying, on day one, that that's how you measure what people want.

"You want a lot of things. You want an A in this class. You want a trip to Hawaii. You want a tasty lunch. And you want all of those things ... 'a lot.' Well, I'm economist, I'm trying to be a scientist. I can't measure 'a lot.' What I can measure is - what are you willing to give up in order to get the things you want?"