But you can't say X + Y is better than Z (by itself)
I mean, yeah, you absolutely can. Especially when X + Y and Z are both common configurations, and using X or Y by themselves is uncommon or a known bad setup.
Sure, you can't be certain which of X or Y is making the differences in the comparison, but the comparison can absolutely be made.
You can save quite a bit by getting a refurbished Pixel - looks like the cheapest "Google certified" option (so it comes with a 1-year warranty) is a 6a for $250, which is nearly half off MSRP. I've been using my 6a since launch, so it's been going for 3 years now and I have no desire to upgrade.
You can definitely get cheaper smartphones, but $250 for a 6a feels like a pretty big bang for your buck.