I understand that copies of the Codex Seraphinianus are quite pricy, though not in the 600-ducat range.
Cheapest copies on Amazon are 80 bucks. But it is all over the internet.
It should be noted that there are two different kinds of “we don’t know”. One kind is where we can’t think of any explanation. The other is where we can think of many explanations, but we don’t know which is correct. The Voynich Manuscript seems to fall pretty squarely in the second category: It could be that some fraudster with a modicum of artistic talent knew that some particular noble would pay well for books of esoteric knowledge, and so faked up a book of esoteric knowledge that he could sell. It could be that the author was crazy, and he thought it all made perfect sense, but only in his head. It could be that it was real secrets, encrypted in some real code that was only known to the author and possibly a few others. All of these explanations are possible, and we can’t really rule out any of them.