After poking around a bit, I found another estimator, TCK Publishing's Amazon Book Sales Calculator (using the sales per month entry) which yields significantly larger differences.I've been using FREE Amazon Sales Estimator - Predict Your Sales | Rank Calculator to look at what the ranks mean. It surprising sometimes how big and how little a jump from its low of #6200 to a recent high of 1900 is. That's a 20% increase in monthly sales, if it continues. That's significant for a typically low margin industry. However, that is only 90 exrta books per month, if the calculator is correct.

For reference, 5th ed PHB at #30 earlier today is almost double that number of books per month, although this is also a recent high and probably won't last.

It puts D&D PHB sales at about 15-20 times those of the PF2 CRB, and PF2 Bestiary sales at about 40% of the PF2 CRB. (And it puts D&D Monster Manual sales at about 40% of the D&D PHB sales.)

But I have no idea which of these calculators is more reliable...