It's all about the exclusivity
There are other cars with the same amount of horsepower, for even more money. Take a look at the BMW M3, 333hp for 55 grand. The last M3 model had 240 hp and sold for 45k... but just to start, really you should compare this car against Mercedes, which what it really is.
Obviously they sell SLK AMG coupes and convertibles, and again, the price tag is higher. So why is the SRT-6 so "unreasonably" priced? I don't believe that it is.
Also, how can you compare it against a 350Z in horsepower per dollar? Your getting a Japanese ricer car, and your trying to compare it against a German luxury sports car. Obviously the Jap car will have a better dollar/horsepower ratio. But does it come with navigation? 19 inch wheels? A Mercedes engine with 330hp that will last 500k miles? No.
As for Corvette... cmon, Corvette's are so cheesey! Nobody want's an American car these days.. and American the Crossfire is not... We are talking cheap interior, annoying rattles, inferior American parts... Younger people don't want Corvette's, they want something sportier, something newer looking, something more luxurious.... not a model rehashed 10 times.. Something like the SRT-6... I think people who grew up with Corvette's being THE car to have find the Corvette more attractive. And really I don't think most Crossfire owner's are the Corvette type... They are played out. Nobody looks at Corvette's and says "WOW". We've all seen them a thousand times! But I guarantee you drive the SRT-6 around town, you'll be getting WOWs for forseeable future.
Compare it against car's like a Porsche turbos, a BMW Z8, rare cars like that... Thats where I think a comparison can be made. And in these instances, the dollar value kills the competition.
The bottom line is you are getting an awesome looking car, a sick performance machine, at what I believe is a reasonable price. On top of that, the car is very exclusive, rare, high quality in every part, and destined to be a classic. It simply cannot be compared against cars you see every day. (like the Corvette)