That's economy of scale I don't see a reason for it to be more expensive the Apple A8x had a higher transistor count than an 8 core Core i7 Haswell-E CPU.
Also I'm pretty sure that high end SOC's that end up in iPhones, Galaxy S's and the like cost more like the Core-M than the 20$ or so you would pay for a MediaTek SOC.
I've design with Intel Atoms and ARM chips. Atoms are very bloated and require a lot of external support to get up and running. Intel's firmware/boot support is atrocious. They still have PC OEM's in mind. ARM chips on the other hand are much easier/cheaper to design for. The SoC is better integrated and targeted towards a low overall BOM cost. Core M is worse than Atom.
1) they wouldn't have replaced its Core-based Celerons and Pentiums with Atoms to increase profits - and they sell those for $110-$160
2) they wouldn't have licensed Atom IP to Rockchip and other low-end chip makers, essentially pursuing an ARM-like IP-revenue model (for which they would've made peanuts and didn't pan out anyway).
mediatek socs are actually $4(this is a price from 2014, when Intel had to match that $4 price while subsidizing Atoms), and that includes pmic. $20 is a price for a hieng quad core chips.
