from Chryslerweblog.com :

No Dodge Challenger convertible for you
October 8th, 2008
From the article: “We considered a convertible when we were developing Challenger, but the extra weight needed to beef up the car when the top was removed was too much of a penalty,” he said. “And that was with a manual soft-top. A power-retractable hardtop would add even more weight.”

There’s an even better reason not to add the weight — and cost.

“We didn’t see a huge market for a convertible that would justify the vehicle,” he said.”