Most Cobra manufacturers makes their own frame and body. The cost will depend on what you want to spend. You can get a kit that you do complete assembly, or get a roller that only needs motor and trans dropped in.
Factory Five is probably the largest Cobra replica manufacturer out there. They were originally built on the concept of a donor '87-'93 Mustang, where you can use the motor, trans, dash, wheels, etc... and put it onto their tube frame and body (and other parts). Very cost effective and tons of fun. You don't have to use the donor Mustang if you don't want to, but you can. I have a Factory Five and couldn't be happier. You might want to check out
www.FFCobra.com, which is a forum devoted to Factory Five. Or
www.factoryfive.com But there are LOTS of kits out there. Even though I'm a firm believer in Factory Five (FFR), and would recommend to anyone to buy one, don't limit yourself. Check out a bunch of them, and decide for yourself. You'll come back to FFR.
Steve