Last month at a local regular developer meeting someone came with a 75 EUR (about the same in USD) windows 10 tablet and it was ok to him. You can try any of these and ditch them, if you're not satisfied. From the OS perspecive OEMs just pay a fraction of the price an end user pays for Windows, so you mainly pay for hardware. Anyway those are very cheap, to cheap. I just googled prices for displays only, bare display modules you may use for a raspberry pi start at these prices for 8inch displays, so you mainly pay the display price. Surely manufacturers get any hardware module cheaper, maybe even for less than 50%, but it's still not shedding a good light on such cheap devices.
Since VFP only needs a few MB RAM your needs of hardware rather are defined by the OS, and I wouldn't run Windows 8 or 10 on less than 2GB RAM. Notice the GB specification rather is about the "HDD", mostly slower SD card like flash memory, much slower than a decent SDD. So this also isn't very fast for database access and while the stability of the flash memory got better, it's still likely the first hardware module to fail.
The RCA somes with 2GB, but overall I think you would like to spend a bit more, at least $200, ideally $300-§400.
If you think of selling bundles of tablets with your software it may well be worth finding the cheapest device that'll run your software smooth enough, the speed of the flash RAM will be your bottleneck (besides good programming and indexing) so I'd concentrate on that, it's hard to get at the specs of that flash memory, though. And forget to spice it up with the best SD card you may add external, the bus speed is your limitation about the SD card slots of low price tablets, besides those high speed cards might well double the price again, as you can easily spend $100 on a 128GB SD card only, if it's 600x speed - and you won't get this speed.
Bye, Olaf.