Wow, that’s a rare beast, Tom. It’s also a bit of a unicorn and, since no two converted Patrol tippers were the same (more or less) there are no hard and fast rules on what they’re worth second-hand.
From the sound of things, yours is in very nice condition and has low kilometres, so they’re both good selling points. But from a buyer’s point of view, I’d want to know that the vehicle has been properly engineered (as these were converted to tippers by third party modifiers, rather than Nissan itself) and that it has all the paperwork necessary to obtain a roadworthy certificate and be legally registered. What your insurance provider will make of such a vehicle is up to them, but it’s another consideration for would-be buyers.
As for a price guide, the only similar vehicle I could find for sale at the moment was around the $20,000 mark. But this is a classic case of a vehicle being worth precisely what somebody else will pay for it.