I don't know whether it's because twenty dollars is the largest bill most Americans keep in their wallet, or some other reason, but that seems to be the magic 'dividing line' between the 'casual' games and the longer (and presumably better) ones.
the following might be factors:
diablo games are 3rd person isometric graphics, not 1pv 3pv 6 DOF (flightsim or shooter with roll) correct?
diablo games are more action oriented than some other more hard core fantasy rpg titles, correct?
the magic $60 price point in america seems to be reserved for state of the art hard core releases. something like tomb raider or assasins creed being as close as one might get to lighter/casual action in such titles. and you have to admit both of those games have evolved over their various releases into rather hard core state of the art examples of their particular types of games.
at the other end of the scale, $15 is another magic price point - or you might say its a nice one that lies between two other magic price points: $10, and $20.
$10 is special, because it denotes lack of or limited value. any game that's $10 or less can't be that big, no matter how cool, or it would cost more. or it may just not provide more than $10 worth of gaming entertainment value to the user.
Note that at this low price point, one must start putting qualifications on what type of game we're talking about here. $10 is quite reasonable for an arcade, puzzle, or card game. but lets stick with "bigger" games only.
$20 is special, because at this point, its no longer an impulse purchase. $20 bucks will buy you a pizza and a six pack! <g>. (well, depending on the beer you drink...).
so at $20, the user starts thinking in terms of value, and also opportunity costs: "what else better could i do with my money that would make me even happier?". after all, as Desmond Morris said in The Human Animal, "After survival is ensured, it's all about enhanced play.". so they start thinking twice about how to use those 20 hard earned survival points (dollars).
this concept can be translated to any market worldwide. over the years, Rockland has sold internationally from hong kong to berlin, the LONG way around the planet. Thats why Rockland runs on Hawaii time. which is a real B---- for me, as i live on the east coast.
but in any market (people are the same everywhere) such price points will exist:
* too cheap - cant be that good a value, even if its a cool game. its either not that good, or not that big - or both (too many of those out there - makes me wonder why they even bothered writing it. perhaps as a learning exercise?)
* kind of pricey - got to think about that one.
* looks cool, not too expensive for what you seem to get.
* expensive state of the art stuff - NBA2K. but i gotta have it! i'm a SUBCRIBER! <g>
they may vary depending on the availability of titles for that market, local market conditions (local economies), and what other things compete for entertainment dollars in that market. and these three things can vary widely between markets.
if your not in the us market, think of your own local market in these terms and see if you can't guesstimate what the numbers would be for your market. just put yourself in the average target user's shoes.
and note that the us market is not the world market. historically, the us had been somewhat heavier on action titles, whereas europe for example is known to produce many high quality simulations, some of which Americans never even get to see. And Asia has yet a 3rd flavor of game style and culture it adds to the mix. in the end its a big wide world out there, and there's lots of potential target users from all markets for all gametypes.
so then it comes down to good old capitalistic competition, build a better mousetrap, survival of the fittest. and this contest for the users dollars is played out simultaneously for each game type. so if your making a diablo game, your competing for diablo user dollars. and if your making a caveman sim, your competing for caveman sim user dollars.
fortunately for me, after 14 years, Caveman is still the only caveman simulator on the market. in fact a game like it in any setting is difficult to think of. then again, i have yet to finish DLing sims3 from origin. this is a basic comptative strategy of Rockland, to try to dominate in niche markets where you're the best, or preferably, the ONLY player. needless to say, being the only player in a viable market is not easy. and not always maintainable.
Rockland built the worlds first Star Trek flight simulator. the gametype eventually became so popular, Paramount got into it. To be the only player, basically you have to be unique - and cool - or there'd be no market. this requires innovative game design. IE you cant just make Y.A.S. (yet another shooter), as there's already one (or a million) out there. the game has to be unique. if they want to play something like that, you're the only game in town (no pun intended <g>).
dungeon keeper might be an example of a unique game like this. the concept is cool , D&D and you play the monsters! and there was nothing like it out there at the time, despite that playing the other side had probably occurred to D&D players for 20 years by then.