I think Spak-Spang brings up some truth. I mostly agree but it's possible to play devil's advocate.
There so many alternatives to buying new games (ie. buying similar old games, renting games, borrowing games, and, GASP, pirating games). You can't rent/borrow dinner. You can't rent/borrow a movie until it comes out on DVD. And you shouldn't rent/borrow a date. And just as with anything else, people want more value for their dollar whenever possible. A game may be a good deal comparing it to other forms of entertainment but $40-$60 all at once isn't something people are willing to part with all at once. Plus I think people are more willing to spend money on a date, and most people still don't consider playing a game a date activity.
It's hard to say how many people don't finish their games but enjoy them, or how many people replay games and how many times they do. So in that sense it's definitely hard to quantify. But I still think a longer game is better than a shorter game, and I don't need numbers (specific ones, anyway) to know that.
I've heard that you can't use numbers to quantify joy, as much as one of my favourite university courses told me (PHIL 325 - Risk, Choice, and Rationality...all about assigning numbers to choices to deduce what the best choices are). Or shouldn't, anyway. However, sometimes when I do, it just makes me want to spend more money. It just depends on how I look at them.
If I can buy two games with a day's worth of wages, anytime I want to buy a game and see a $50 price tag, I go, "oooh, that's half a day of work." If I think I had a recent half day of work that was pretty easygoing then I'd be tempted to think it was a good deal. Well, as I said, I don't buy games that often but that raises the temptation. If I think, "oooh, that's half of a tenth of a thousand bucks" then I wouldn't want to buy it. Though that's more numbers...maybe you're right, numbers can't quanitfy joy, but rather dilutes it.
Maybe ECON 101 (opportunity cost) and PHIL 325 (assigning numbers to choices) aren't courses to totally live by, interesting as they are.
I don't know if I made any good points or counter-points there.