I don't think there's a cost/performance "justification" to be had, the pricing of these cars is somewhat accurate (if not a bargain) to real life, because they're such rare and collectible cars. Look no further than the Pebble Beach auctions. Even hypercars like the LaFerrari go for 3-5 million nowadays due to appreciation and collectibility post-production. A local in my area has an extremely rare (as in, I believe only two still/ever existed) Aston Martin DBR2, and was either bought or last valued at around 19 or 22 million USD (don't remember the exact stat, but it was that kind of range).
When it comes down to the game, however, I think the problem lies in the economy system, rather than the pricing. If there were larger payouts or other incentives for good performance in Sport Mode (talking like over a million Cr. for race wins in the top DR/SR categories), then not only would there be a more ravenous playerbase in Sport Mode, but the "Unicorns" that have slowly started creeping into the game would become much more attainable, and "fun" to work for. Going in circles (literally) for days (again, literally) definitely isn't a good way to go.
That said, I'm about 3.5 mil away from attaining it (and I think I may actually go for it cuz if nothing else I think I may want to try to throw on a livery similar to the DBR2's that I mentioned above), and I can compare it to the XJ13 and Mk. IV Ford that I already have. I'm betting it'll be slower than the '60s Le Mans racers simply from it being an older car, but it'll be interesting to see how it stacks up against things like the old Shelby, Alfa, or Ferrari GTO.