I'm sure the inflation would determine the absolute numbers, but not the amount relative to New World gold and silver - and there's no mention of any significant income from the colonies outside gold and silver.
And why would income from taxes be greater via inflation?
I think that a definite answer in this regard would come from the original sources, where
Rise and Fall got its numbers from. The way I see it, the information could come from colonial records of what’s being shipped off, or tax records from the peninsula. In any case:
What you basically have are separate economies, with one (the colonial) sending off tribute to the motherland.
During the XVI c. the “income” from the colonies would be shipped off in; “cobs,” the everyday coinage of the colonies and a terrible attempt at imitating the monies of the Peninsula; or in ingots. The lack of a consistent coinage operating on both sides of the pond with accurate weight and size standards might also affect some of the numbers.
Here’s where the funny math comes into play; let’s say that the Mexican authorities ship off 1000 ducats to Spain, from what they collected/minted. Upon arrival in Spain, the cargo would be melted down and reissued as actual coinage with the appropriate size and measurements. Now the money is in Spain, where inflation has escalated the prices. As a result, those same 1000 ducats now buy a lot less than they could have back in Mexico.
Dealing with merchants only, taxation would be proportional and at rates; not a set amount per product sold. If in 1500 a pair of shoes sold for 5 ducats, the government taxing at 20% could only get 1. With the inflation in 1550, a pair of shoes sells for 50 ducats; the revenue in tax would now be 10 ducats.
The difficulty here lies in realizing that the economies were not as heavily intertwined as they are today. The surplus of metals could affect Spain itself and its citizens; the tax code might not change, but the value of the money had. People paid more for goods with a devaluated currency, and the government, keeping taxing rates at a constant level, would get more ducats which had a decreased purchasing power in Europe, but were nominally still the same “currency” as that which came from the New World.
I do have to admit that the second half of my response is only speculation. But in terms of the absolute numbers, we’d have to know with certainty what the levels of inflation where in Spain itself, and the colonies.