Besides the already mentioned point of getting one refurbished, Pixels tend to get really cheap towards the end of the yearly cycle. At that point, they were mostly going to make money from you using their ecosystem and then you are sticking it to them by installing GrapheneOS :p (probably they don't care).

E.g. a new Pixel 9a is currently 369 Euro in The Netherlands and 367 Euro in Germany. The Pixel 10a will be released soon, but the 9a will run GrapheneOS just fine (same SoC except modem as the vanilla 9).