When they charge 80 or 90 dollars or whatever, people do tend to have their expectations rather high, yes. This is justified by the cost. I dont think a "decent time waster" should cost that much. It is competing with alot of other, cheaper games.
I dont know why people frequently ignore price when it comes to judging a game (this applies not only to diablo). When I judge a game, its always
quality = (price > 1 ? entertainment/price : entertainment)
$90 Dollars. . . not in America at least, it's $60 here, (or at least was, considering I'm rather out of the market now that I own one copy). That's your standard price for a new non-indie game release. I'd say I got my money's worth. Also, your supposed mathematical formula doesn't make any sense. You can't have both "=" and ">" in the same equation, "?" Isn't a function and ":" is also not a function.
So ultimately we have
Quality = Entertainment/price, which I'm going to assume you meant
Quality = (Entertainment gained from given object (Diablo 3)/Entertainment gained from other sources within the same budget)
So in my area, I could go see about 3 movies, approx. 5 hours of watching movies we'll say. I could go out for a couple of nice dinners with my lady friend. I could spend it all for one night of drunkenness. So basically the equation will, for me (I repeat) boil down to
Quality = (Entertainment gained from Diablo 3/Entertainment gained from 5 hours of movies OR having nice dinners with my fiancé OR getting passed-out drunk).
Having spent something like 50 Hours (low estimate) in Diablo, I've spent at least 10 more hours being entertained than movies, of course I like spending time with my fiancee as well but the cost of Diablo has not cost me as much time, so the opportunity/cost is mostly a monetary cost so I will discount the second option, I just included it for reference to popular activities others might be comparing it to. Let's just settle it down to movies because I like those the most so it'll provide the best comparison
The next step would be assigning an average entertainment value to both activities. I'll be using a 1-10 scale, with 1 being the most boring (watching TV) and 10 being the most entertaining ("time" with my fiancé *wink wink nudge nudge*) In that scale I'd give Diablo something between a 5 and a 6, so we'll be going with a 5 just to go with the lower estimate, again. The very best movies I would give a 6 in entertainment value, most being a 4. So I'll even give the three movies I could see the benefit of the doubt and give it a 6.
The formula for entertainment should then be, Hours of Entertainment given/Average Entertainment Value.
Diablo 3 Entertainment = 50/5 = 10
3 Good Movies = 5/6 = 5/6
This doesn't seem fair because I'm only accounting for seeing the movies once, when I've played through Diablo 3, 3 times so I'll adjust it even.
Diablo 3 Entertainment 18/5 = 3.4
3 Good Movies = 5/6 = 5/6
So ultimately, using your formula
Quality = 3.4 / (5/6)
Or a 4, Diablo roughly scores a 4.
However that means nothing because it's not part of a scale or anything. Ultimately the entire formula is rather useless and entertainment is all imaginary, perceived differently by each individual, trying the turn entertainment into a mathematical formula is... well stupid. I'm just as stupid though for taking the time to write all of this out just to prove the point though. As is quality, it is all perceived, subjective really with regards to this matter.