The PineTab is an upcoming $100 open source Ubuntu Linux Tablet being created by PINE who are known for their low cost Pine64 single board computers, Pinebook Laptop and Linux based PinePhone. The PineTab is not yet for sale, and they have just announced their intention to begin taking pre-orders in late May, and that the first production run will be a limited quantity pilot production intended only for early adopters.
What's interesting about the PineTab is that they are advertising that they are working on expansion options, with one expansion module being an RTL-SDR. It seems that the expansion module will allow cards to be inserted internally, keeping everything tidy on the outside. Apart from the RTL-SDR, they will also offer LoRa, LTE (with GPS) and sata SSD add on cards.
The standard specs of the PineTab are shown below: