I'm not sure, but I think the electronics for micros cost more than the electronics for FT planes. It's a supply and demand thing, I think. Like how HO scale trains cost a little less than N scale. N scale is half the size, but HO is much, much more popular. Plus more details means more money.
For cheapness, it needs to be a balance. You need to be able to use normal sized Rxs, because the micro ones cost so much more. You need to be able to use standard ESCs, but low amp rated ones for maximum cheapness. Then use a small motor, again for cheapness. Then I think 500mah 2S and 3S batteries are about the cheapest. 9gram servos are the cheapest due to popularity.
Although, the micro stuff can be only a few bucks more, and whats a few bucks here and there?
I dunno. I was looking into it, and I did find this cheap
micro Rx, but I think it's an exception. And I found this
prop, motor, ESC combo for $22. But you can buy a Hextronic 24 gram motor for about $10, and an ESC for the same price.
I know somewhere on this forum someone made a profile indoor 3D plane using more standard sized electronics than using micro stuff. He wished the plane was slower, but it was close to being a micro indoor plane. I think something like that is the best route for a cheap scratch built micro plane. Small, but not technically micro.