Romothil It depends on which factors you want to weigh more heavily.
The cheapest way to try GrapheneOS is probably a used Pixel 6a (from eBay, etc.) -- go for a large-volume seller with a good return policy who is advertising "factory unlocked" devices. Please note that the GrapheneOS project does not recommend devices before the 8, due to shorter remaining support life and fewer security features. But that's the cheapest way to try, as long as you're willing to switch devices in around two years.
The way to pay the most money is to get a Pixel 10. Assuming GrapheneOS support works out, which now seems likely, a 10-series device should have the longest support life going forward.
In between there are various tradeoffs based on battery size, screen size, etc. I think some Pro devices have a built-in thermometer? But since you didn't mention any factors like that, it's not clear how much advice can be provided.