That's pretty much it right there. It's rare as f**k, and has strong racing heritige, therefore can fetch a pretty penny. Also I don't think a lot of people would be purchasing such cars as an investment, because if you have that sort of cash to blow on a car that will be moth-balled, no doubt they also have money in term deposits, property and the share market.
Aus-delivered R32 GTR's are rising in price too