Generally speaking, demand defines value, not production numbers.
If only two of a variant are produced, but only two people in the world want one..........
This is true. That along with "overall" cost. When I was looking at buying my very first Rover one of the main things I looked at was the cost and availability of parts, especially since I'm in the US.
The more uncommon the Rover, or any vehicle for that matter, the harder it would be to get parts.
I know the demand for specific styles and configurations have got to be different there in the UK than here in the US as well. Rovers are so uncommon here, especially in my area, that Rovers North actually has a monthly contest called "Spottings". If you see a Rover in a US advert you submit it. If you are the first to submit it, or one of the first, you win a mug.
The SIIa 88 is becoming really popular over here for various reasons. To give you an idea, within the past year or so an article was circulating on some US based internet magazines stating that the SIIa was THE "quintessential" overland/safari/adventure vehicle. The early 2a and Station Wagons were pointed at the most. Following in that article's footsteps three magazines, who's target demographic are young, affluent suburbanites, ran Series Rovers on their covers and as part of the feature article. The two that I remember are "Guns & Gardens" and "Country Living".
Honestly, I never realized just how popular Rovers were becoming here in the US until about Spring of 2018. I was on a forum and somebody asked what everybody's idea of the perfect overlanding vehicle was that was somewhat easy for the owner to maintain and work on themselves. I, of course, said the Land Rover Series. A guy commented something like "I wish - I'm not rich, can't afford something like that, I had to settle for an LR3". This got me to thinking about what they were selling for now so I checked. I was rather shocked. If I were to try and purchase a '63 SIIa 88 Station Wagon, in the same condition Grover was in, today I couldn't justify the expense. When I looked at those within the same price range I paid for Grover all that popped up were rusted out relics and you'd be lucky if it had an engine that turned over. So far I've been offered 3 times the amount I paid when Grover was halfway through the rebuild. Now that I'm almost done, the offer has gone up considerably.