1. Apple is probably producing their own ARM CPUs at a much lower cost than what Intel is charging them for the CPUs used in Macs. If an A12 costs Apple 20 dollars versus an Intel CPU that costs 150 dollars, that's no small change for low-end devices (ex: the so-called low-cost MacBook Air replacement). It would also put Apple in control of their Mac updates, not having to rely on Intel. They had that problem with IBM and the G5 and now history is repeating itself. But this time, they're taking control instead of trying to rely on yet another third-party CPU manufacturer.
2. Apple's ARM CPUs require less power to accomplish the same task than the Intel low-power CPUs which are full of transistors used for legacy compatibility. This would translate in either more hours for portable devices, or lighter devices that run the same number of hours because the battery could be smaller. A smaller battery would further lower the cost of the device. Less power required also means less waste heat from the CPU, which in turn requires less cooling capabilities.
3. We've seen Microsoft run x86 applications on ARM CPUs. Apple has much more experience with this, so they would introduce "Rosetta 2" for the ARM-powered Macs. Not everyone edits videos, not everyone requires lots of processing power. I'm still using my mid-2010 Mac mini and the only time I feel like my Mac is too slow is because of bloated websites wasting RAM and CPU on dozens of Javascript librairies.
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That's not a valid comparison. Apple uses Intel processors, so you need to compare with a PC build that uses the same parts as closely as possible.
Otherwise, your argument is simply "an AMD build is cheaper than an Intel build", PC or Mac doesn't even matter.
