I was worried that someone might read that Texas's per-capita spending is higher than California's and draw the conclusion that this is a big reason for why California's schools suck. I've heard a lot of people make arguments like that. "Small Country X spends so much more per capita on Y! (Implied: That must be why Y sucks for us, and our lives would be better if our country would spend more on it.)"

So I gave one reason why the returns California and Texas get from their respective textbook purchases are probably not proportional to their per-capita budgets.

This was easy to Google up. There are more recent figures.

I'm speaking to the US audience.

Do we?

Yes. I don't have rigorous data in front in me, but as a society we've become wealthier over, say, the last 60 years. In every community I've every been a part of, talked with others about, or reviewed data on, taxes for schools, as percentages of property values, income, or sales, have gone up. Greater wealth * higher tax percentages = more money/resources spent on schools.

And declining since when?

Though I'm skeptical of using test score data, the US results haven't been pretty from at least the 1970s onward. Anecdotally, however, this seems confirmed by consensus opinion. Education quality and loss of competitiveness of the median high school graduate relative to the rest of the world has been a continuing political issue for decades.

And how much?

There's no way for me or anyone to precisely quantify either part.

As for your skepticism, I simply assumed that readers knowledgeable about modern US history could appreciate the truth of my point based on the weight of their own observations.