I am reading a book, "The Art of Warfare in the Age of Napoleon" which has made the above claim, at least during the 18th century. However, this is a staggeringly high percentage and I find it hard to believe. Is it true, and if not, what did Prussia actually spend on its military?
Additionally, what did other European powers of that time (18th century) spend on their militaries?
During the entire 18th century? No. No way. Might they have gone up to around 90% in the reformation of the army following the the Brandenburg recess of 1653? Yes, but that is a rather meaningless comparison to make in modern terms. You have to keep in mind that the "budget" of a state in the 18th century was quite different from what it is today and following the recess Frederick William had 530 000 Thalers to spend.
The highest budgetary estimates I've seen for the military spending of Frederick William is around 50% but I suppose it depends on how you measure state expediture. Anyway, I did a writeup about all of this a few years ago. Here you go.
What's the source he sites for that claim?