That's a question I wrestle with. Here, they argue that Minecraft is a good example of a game you get your money's worth from. I don't play Minecraft, so I don't know. On the other hand, I also felt like I got my money's worth out of several short games I've played too. The sweet spot for pricing a game is difficult. This is going to be a bit of pseudo-theory, and it is also relatively light, but, long.
If you never play it, it is a waste of money. Buying a whole army of Warhammer minis is a waste if no one plays, unless you like to just paint them. In fact, some table-top RPGs can be fun just to have the books and read the fluff. So, I'm not sure a good way to measure those. But, let's move on to digital games.
Let's deal solely with games that have a definitive end. Let's ignore MMOs or games like Bejeweled or the like that are iterative puzzle/dungeon crawling games. There is, perhaps, a way to determine when you've got your money's worth out of those games. If Susan gets bored with it in three hours and gets distracted by nylons and lipsticks and invitations, is it fair to knock the game, if Lucy has been a loyal fan for years? Probably, if you are Susan.
I have a general theory. I take my hourly wage; then, I divide that into the cost of the game. If, I make about $1 an hour (for sake of math), then I buy a $5 game, I am looking at roughly five hours of my life spent to purchase that game. At that point, I need to figure out what X hours of work should be able to purchase Y hours of entertainment. This gets hard fast.
I can go to the museums in D.C. for, literally, maybe $5 total, in Metro tickets. So, in my example, that is five hours of my life, but a video game is about $50, a larger chunk of life. The cost ratio is different with real wage numbers, where $5 is immaterial in the long run and $50 is a purchase I need to consider, but is not more than a week's paycheck (as if I were making $1 an hour). So, as your pay gets larger, differences in costs get smaller. This is why rich people have yachts; they spend less time to afford nicer things. Because the world is unfair.
That trip to D.C. can be a whole day, plus another two or three hours reviewing pictures and blogging about it. That's a very positive toil to fun ratio (even at five hours of work for nearly 12 hours of fun!) Imagine the toil to fun ratio when that $5 is negligible. Likewise, going to see a movie is, what, $15 for a two to three hour movie? That's a bit harsher of a ratio, and if you include travel time, etc., it quickly becomes a negative ratio, where I spent more time earning the money needed to have that fun. In fact, in our initial example of $1/hour, it starts as a negative ratio. This is why you so carefully rationed out your allowance/money earned by chores/raking leaves/etc. when you were a kid: You spent more time to earn so you wanted more fun from earnings.
A bad movie makes you feel like you wasted your money, even though you, allegedly, received the same amount of entertainment. So, the Y variable probably has some exponents and the like we have to figure out. These differ for different people (in fact, the simple variable Y, total entertainment time, can vary for some forms of fun. Movies have a set amount of your life they'll eat up in one viewing, game play can vary. For example, I can probably speed through King's Quest 5 in 3-4 hours, if that. A new player may need double. So, Y changes.) Unique experiences tend to be more valuable (a larger Y value), while repeat experiences tend to have a lower Y value. You may shell out $15 to see a new movie; but you are much less likely to see it again. Unless it is really awesome, or you value seeing movies very highly. Same thing with a game; it is rarely as fun as the first time.
Now, what about found money (like gift cards and the like)? We actually value that sort of money less than what we worked hard for. Not in a dollar for dollar ratio, but if I find a $20 on the ground, I'm more likely to blow it since I don't really own it. This impacts the relative weighting of X.
X is equal to the amount of time, effort and dollar cost of a given activity. A trip to the zoo has a very low effort and dollar cost, but it may have a sunk time cost in getting to the zoo. Note that the time involved in the toil variable is not part of actually playing an unfun game or unfun activity. It is what you have to do to do the part you like. So, if you hate the actual painting of minis, it shifts from the fun to the toil category.
Y is the amount of fun time, the extent of the fun and the lingering pleasant feelings. That third one is important; it is why people are willing to spend a large amount on experiences (such as dates and weddings) that they would not spend on other things. Taking the cute brunette out to a fancy dinner seems silly, but the fact is that you will both have a pleasant evening and it will leave residual happiness. With games, that's the sort of excited buzz you get when you scan over forums after you beat a game or go to the spoilers forums after a chapter to go: "Whoa."
Which brings us back to the question: How long does a game have to be to be a worthy investment? Like any good economist, I will piss you off and say: "It depends." Most people can do a good gut check and figure it out for themselves. There's no real good heuristic that can apply to everyone, even my most basic (Did I work more hours in my life to pay for this than I will enjoy it for?) Some people, for example, might be willing to pay more for tickets to a concert, like, say for the Legend of Zelda symphony.
This is a really hard question, that I am probably over thinking. But, what do you consider "worth it" and how do you measure it?
I am making progress through my rework of the Dragon Dice rules. I realized I had... forgotten about Firewalkers. That is sad. Poor them.