"Purchase price" is not the same thing as costs and "revenue price - purchase price" is completely different from profits.

With a broad enough definition and inclusion of other, unrelated prices at that negative part, one can claim that in a time-frame of several years, both average to nearly the same. But that doesn't make them equal, and if you go read those tax-codes, you will discover the ones being discussed add to very different figures even on the long term.

I'll just add here another encouragement to go out and get an understanding of tax codes. Odds are they will be relevant for everybody that frequents here at some point on their lives, and odds are that if you don't know them, you'll only discover their relevance after the fact, when they have already caused you some problem.

Yeah. To be pedantic maybe we should say "a tax on gross profit". Regardless, it's closer to taxing profit than what most people mean when they propose a revenue tax.