If you’re okay with an Amazon Fire tablet those are hands down the best tablets under $100. Very few name-brand tablets sell for less than $100 (though I’ve seen refurb 10" Lenovo Tab 4 for a bit over $100).
I’ve had a few RCA and off brand tablets in the $40-80 range and all of them had disappointing performance and compatibility issues with typical Android apps. The budget line-up usually has low-res screens (1024x600), small internal storage (8GB), and buggy OS builds.
In the end I was happy to find relatively high-quality hardware and stable OS on the Fire tablet line for typically less than $50, even if you have to install the Google Play Store manually.