Speculative fraud hypothesis

[dexdex]

The fact that Rudolf paid 600 ducats for it has been brought up many times in this context, but, of course, it is completely irrelevant.
The fact that he bought it does not say anything about whether it is authentic or fake. 

It does not matter whether the manuscript was or wasn't authentic. In fact, it doesn't even matter whether Rudolf believed it was authentic. The fact that he spent the money shows -- dare I say "proves' -- that there was indeed sufficient motivation for someone to spend the time to make a manuscript like the VMS and that it was not necessary to make it of any higher quality than it turned out to be. (Also, not only did Rudolf reportedly pay a lot of money, but Marci seems to have brought up the point of the purchase in order to bolster his case that the manuscript was something of value and therefore worthy of Kircher's time to decipher it.)

Koen's point that Rudolf's willingness and ability to pay a lot of money for it in his timeframe is removed from the original motives of the (much earlier) manuscript producers is true, but I'm not sure it is therefore irrelevant.  Just how much do we think human behaviours change over time?   How little time between the production and the buying is necessary before it IS relevant? If there was demonstrable evidence that the manuscript could convince a wealthy buyer to part with a lot of money 400 years ago, is it that much less evidence that they would part with it 600 years ago?

I believe there's a misunderstanding here: people's motivations haven't changed, and it would be a plausible reason to do this if you stood to gain what the sale to Rudolf did. However, at the time of Rudolf buying the book, it was visibly 200 years old: an esoteric antique (because it was 200 years old by that point). This considerably increases its perceived value to a fan of esoterica. When it was made, any interested buyer would be savvy enough to see it wasn't some old and beautiful tome, but a contemporaneous document. To my knowledge, there are no signs the original manuscript was intentionally made to look older to increase value as a "sell once" scam. 

This means you cannot use the Rudolf sale as evidence that it would be potentially worth 600 ducats in the eyes of the creators and/or their potential mark. Because it was new, not an antique. A reasonable assumption is that it would be worth far less, even discounting the likely fact the 600 ducat sale was a bunch of books, not just one.

Also, another thought: that VMS is not that incredibly ornate works to its advantage as a prop - it's not so expensive that you'd want to steal it, and its only 'value' to the mark is through the book's owner claiming they can decipher it. Of course, it doesn't explain why so much effort went into creating the text to have the characteristics it does, but at least from the appearance standpoint the level of care and professionalism (somewhat amateurish, but decently made; some sections of much better quality than others; no religious imagery that might draw any religious leader's ire) fits a quack's prop. It isn't all that impressive for a contemporaneous book being sold as some rare artifact.

You are correct in saying it could have been someone deluding themselves into thinking the book would be worth much more - people's motivations are not always rational. But then we run into the problem of the consensus about multiple scribes, as well as the fact that the creation of the Voynich, amateurish as it is, still involves someone very well educated. These considerably lower the chance of a delusional person making a hoax for sale.