Well, no one says it is worth this much, because things are worth what someone would pay; if Belardo lists his zip drive for $5000, it doesn't make it worth $5000 when no one buys it.
The production version, in working order with it's accessories looks to be worth about $100 or so, given *sold* prices on eBay. I can't find production numbers, but if they made about 100k of them and they are worth $100-$120, how much is a prototype that is one of maybe only six worth?
Check out what some of the 'collectible' Sun and Silicon Graphics and NeXT, etc SELL for on eBay... maybe most pertinant listings in this case are the following:
A prototype working Mac 128k *sold* for $100k
A working production model Max 128k for $1300
That has the gap between the original and protoype at 77x, so if the production unit is worth $100 or so, the prototype might be worth around $8000...