I saw Nvidia is about to release a $60 board (Jetson Nano 2GB).
The CPU performs pretty poorly, but you'd really want to get it for the Maxwell 128 core GPU.
Although it performs somewhat similar to a 6 years old GT 730, you can stack a lot of these units together.

Product page:
https://www.nvidia.com/en-us/autonomous-machines/embedded-systems/jetson-nano/education-projects/

I had my original Nano running its gpu on Seti when it had work. I currently run the Einstein BRP cpu app on the gpu. You have to have access to the sources to compile your own app or have somebody hand you one they have already compiled.

I saw that announcement of the new 2GB Nano. I checked to see how much memory the BRP app uses and it would fit fine within the 2GB model and have room to run one cpu task of small memory footprint.