Between 2003 and 2011, fewer than 16,000 XLRs were made. Part of the appeal of the Corvette is that it offers tremendous power and performance for far less money than its rivals, and by offering virtually the same car with a nicer interior, part of that appeal is taken away. Was it fun to drive? Yes, of course. Comfortable and well appointed? Absolutely. Worth the $30,000 premium over the well-established ‘Vette? Apparently not. This is a lesson that Cadillac has been slow to learn — it’s currently grappling with a similar situation with its ELR and its close relations to the Chevy Volt.

Well, they had two choices, exclusivity or value.  They definitely were awesome for performance/exclusivity, but unfortunately those two things beat up on practicality.  I'm holding out for an ATS-V, but still worry they are going to price it out of the market.  

It's an old lesson, never learned:  http://en.wikipedia.org/wiki/Cadillac_Cimarron

In the tradeoff between exclusivity and value, how does a car company figure out where the sweet spot is to get enough business to make the car line successful?

It seems like an opaque art to me.