Shameless plug, my startup wholesales a 3G Android smartphone with 4GB memory running on a Spreadtrum chip that costs $30: https://www.voxsupplychain.com/shop/users/#/shop/product/and...
We have found that there are some natural dividing lines on low-end Android specs. If you cut more corners, it really impedes the user experience.
For example: I use the terms flash memory and flash storage interchangeably. The other way round, RAM can be thought of as temporary storage that is lost when the device is powered off.
Lowest we could go for a SC7715 was about 35$.
Also, we work directly with carriers and get wholesale rates for air and sea freight to major ports. Delivery to Hamburg by sea is about $0.05 USD per device for 1,000 smartphones.
These phones would make excellent autopilots and controllers for RC cars, quadcopters, even regular helicopters, but there's just a few things missing.
My email is in my profile if you'd like to be in touch.