I agree that the P1900 was the most limited production Volvo ever. However, if memory serves me, it was never actually put into production after a Volvo exec took one out for a weekend test drive and found the fiberglass body lacked enough rigidity for even mediocre handling.
What made the Flathood unique is its true status as a Group A homoligated version of the 240. There were exactly 500 made, to meet the FIA Group A requirements. They were all shipped to Norfolk, VA where an FIA inspector counted them and signed the appropriate paperwork. About 20-30 were shipped back to Sweden to be made into a successful race car. The rest were shipped to dealers across the US.
Volvo was also required to publish a set of Homoligation papers that listed the "approved" modifications and upgrade parts that were available, in very limited quantities, at the time. The aluminum hood and rear wing are still much sought after.
IMHO, what makes the Flathood collectable is: Limited numbers, A nice pedigree (success in the ETCC), Drivability, Unique appearance.
what makes them fun to own is the fact that they are, at heart, a 240 Intercooled Turbo and are as easy to maintain and modify as any other T. It is one of the few collectable 240 models, the other being the '79 GT.
Would I have paid $13K for this car? Not unless I hit the Powerball tonight. But that's between the seller and buyer.

Skip
'93 850GLT
'83 242TI Flathood