It's anybody's guess as to what "IT" is about a certain car that makes it an eventual collectors item.

limited production.
distinguished performance.
racing history.
seating capacity.
high initial purchase price.
styling.
limited public appeal.
fun to drive factor.

These are just a few reasons, I'm sure there are plenty more. But it's not always extremely low production #'s that dictate collectability. For instance, over 50,000 Barracudas were produced between the 1970 and 1974 model years, and just look at what these cars are worth now. It doesn't matter what engine they have, the "million dollar" Hemi Cuda feeding frenzy has pumped up every "E" body's resale value.
The 1969 Camaro is the current "Darling" of the classic car auction community, even though nearly a 1/4 of a million were produced.
So I really don't think total Crossfire production #'s will ever affect its future collectors status one way or another, at least not in any extreme fashion.
I think because the XF is a 2 seat sports car, that's great to look at, a blast to drive, and relatively scarce, it will do well at some future classic car auction 20 years from now. If past auto auction history is any yardstick to go by, I'd have to say an SRT-6 Roadster will be the top money maker to own, followed by the SRT-6 Coupe, the Limited Roadster, and possible the base roadster bringing more than a Limited Coupe. With the Base coupe being the bargain priced "steal" of the auction.

To answer kjypowers question, I believe Ford planned on building no more than 55 thousand "NEW" T-Birds, so they would be considered "rare". Their words, not mine.