[Someone]

On mobile, memory _may_ not be _that_ cheap if you look at what you pay for it in power requirements.

Exact data is hard to find, but, for example, http://www.buildcomputers.net/power-consumption-of-pc-compon... claims 2 to 3 watt per stick. That seems very little compared to CPU, video card or backlight, but your RAM more or less uses that _continuously_. Power usage doesn't go down if you don't use your laptop/tablet/phone, or don't use part of the memory (this may be an area where innovation is possible). So, power drain while your laptop/tablet/phone isn't is used may be dominated by RAM power requirements.

As I said: hard data is difficult to find, so I don't know how big this effect is. I think power usage is an important reason why Apple keeps the amount of RAM in their tablets and phones fairly low, though.