Valuation is vehicle appraising and that is not something that is easily done by most. Is this for insurance purposes or selling purposes? The Z is only worth what someone will pay for it. I think ebay and auction houses will bring you what the "market" thinks of the value for the car because the Z will sell to the highest bidder. If you can find some recent data of sales of Scarab cars as reference, it should give you an idea of what your car is worth. That said, other than the uniqueness of owning an actual Scarab, built by the company, there is not much collectable value (completely my opinion) since there are still a good number of z cars available, they still are relatively cheap, and the available engine options have gone through the roof. Dare I say that some of the daring conversions on this site now more than rival anything Scarab ever produced? That remains in the eye (and wallet) of the beholder.
As said above, dsommer owns a very nice 240z Scarab and might lend some insight into what you are looking for.
I think your car is clean, has most of the Scarab options, and appears very well maintained. My opinion isn't worth much *haha* so I look at your car a bit differently than a Scarab enthusiast, but I'd say your car is worth $9,000 to $15,000. That figure comes from nowhere, really.
Email or PM dsommer.
Davy