Well for one, The games have generally far shorter and smaller level, meaning far smaller time to make the level, there's also MUCH MUCH less items and stuff in the levels (they're also less detailed but let's get to that later), meaning even less time to make the levels. they also avoid stretchign the levels beyond playable area and rather use the skybox/matte's for distance stuff.
The creative process cannot be quantified. A smaller level can be more/less detailed which means the props within could have taken longer/shorter to make. It's all relative.
Secondly, Models, They're far less detailed, and far quicker to make, there's also far less of them, with smaller shorter games on smaller formats there's less unique objects.So far less time spent modelling
Less detail doesn't mean easier to make. Also, with how modells and props are made you only ever need to make something once with a couple texture variations and then duplicate it throughout the level. Sure if you have more "content" then that's a different story but most levels are fleshed out with duplicate models and "clutter" objects.
Thirdly, Textures. Not even close to the amount, and they're lower res, though this may or may not matter depending on the game type, some games the texturs are drawn specifically for the res, but generally textures are made in far higher res than even the full console games can support or even PC and then scaled down. On handhelds though, they often need to be specially drawn at low res in order to have any visible detail on such small screens.
Lower res also doesn't make them easier to make. In fact, its harder to make lower resolution textures identifiable. Which is a counterpoint that you, yourself pointed out meaning that it takes longer to make the textures for handhelds and adds to their dev time, not subtracts from it.
Fourth. Coding, Depends on the game, but at least with Mario it's safe to assume that the coding is nowhere as complex as Halo. but this is a point where the differences aren't necessarily huge
The "coding" is called programming, and it is as complex as needed. Don't forget that games can not only be scripted in an engine but the engine could be made from scratch and regardless of how complex an engine is that is no easy task and can takes months to make.
Fifth. QnA. Smaller games, and less complex games, means far less QnA
Not necessarily. The games get as much Q/A as they can/want.
Add it all up and compare a game like Mario to Halo and you have somewhere between 50 to 100 times less Dev time (counted in man hours, with smaller teams the actual "time" from they start it till it's done will be the same.)For other games the difference will probably be closer to 10 times less. But still a significantly bigger difference in man hours of dev time/resources than the difference in price.
Adding it all up comes down to the game and how much time the art takes to be produced. Art isn't as easy as hitting a button. All styles and kinds have different difficulty levels and nuances. Just because something looks "simple" doesn't mean it is
simple to make. The length of a game, the size of a level and the detail of a model do not directly represent how much dev time it took to be produced.