The "600 ducats" story has always felt like a bit of oral lore to me ("so-and-so told me the emperor paid 600 - what was it, ducats? for this! and he couldn't even read it!"), more than an exact calculation where Marci had access to the original transaction and properly performed the currency conversion. If so, I suspect the most reliable part of the story would be the memorable number 600 rather than the currency unit or the exact circumstances under which this amount was paid.
Two points of clarification for casual readers:
1) 200 florins/yr was the budget for the Imperial Court Library in Vienna. Guzy suspects the manuscript was stored in Rudolph's personal Kunstkammer, which had a number of individual book-related transactions (such as the aforementioned herbaria) for hundreds or even thousands of florin. So, a price of 600 florins is completely plausible, but more likely for a batch of manuscripts than a single book.
2) If I understand correctly, "florins" here are southern German gulden (a silver coin in Rudolph's time), not the better-known Florentine florins (containing ~3.5g of gold). Gulden had long been a standard currency of the HRE, but the term itself was fairly generic and changed meaning several times. Various nations (including the HRE) issued ducats containing ~3.5g of gold, but the exchange rate of silver to gold fluctuated over time, so doing the actual conversion from thaler to gulden to ducats is not straightforward. (I'll defer to Guzy's expertise here.)
Two points of clarification for casual readers:
1) 200 florins/yr was the budget for the Imperial Court Library in Vienna. Guzy suspects the manuscript was stored in Rudolph's personal Kunstkammer, which had a number of individual book-related transactions (such as the aforementioned herbaria) for hundreds or even thousands of florin. So, a price of 600 florins is completely plausible, but more likely for a batch of manuscripts than a single book.
2) If I understand correctly, "florins" here are southern German gulden (a silver coin in Rudolph's time), not the better-known Florentine florins (containing ~3.5g of gold). Gulden had long been a standard currency of the HRE, but the term itself was fairly generic and changed meaning several times. Various nations (including the HRE) issued ducats containing ~3.5g of gold, but the exchange rate of silver to gold fluctuated over time, so doing the actual conversion from thaler to gulden to ducats is not straightforward. (I'll defer to Guzy's expertise here.)