There are a bewildering array of models.
The "PC 2" is bleeding edge, using the H5 chip.
I am pleased by the devices with onboard eMMC since they can ultimately be used without an SD card. All of these have shipping charges in the $3 to $4 range and may require a microSD card (another $6 or so).
The Zero is $6.99 and uses the Allwinner H2 chip (32 bit Cortex-A7).
The Zero with 512M is $8.99 (spend the extra $2).
I bought a pair of the Pi PC to get started. One also has to buy some 8G microSD cards which cost about $5 each, bringing the true cost up to $20. When you consider this, the Pi PC Plus looks quite attractive.
The Pi PC 2 is $20 and uses the Allwinner H5 chip (64 bit Cortex A53). The word is that the H5 is very similar to the A64, for which data sheets are available. I bought one of these in 11-2016.
Tom's electronics pages / email@example.com