I paid $300 for a mint condition 9a about a month ago on Swappa. Obviously the price is lower for other conditions.

https://grapheneos.org/faq#recommended-devices

It’s easy enough for anyone willing try. Grab any Pixel from the last few years, plug it in to a computer, and click buttons in your web browser on the GOS website and watch GOS get installed on your phone. The instructions hold your hand the whole way through.