Well the frame is about the same all you'll have to do is switch out all the panels,trunk ,hood..etc. It would be in the range of $3000 if you buy parts and do it yourself.$5000 to $8000 if you pay someone to do it for you.Buy a used impala body from the junk yard for about $700. Good luck.
The main problem that you are going to have is that the '91 Caprice had squared off rear wheel openings. '94-96 were rounded, so without replacing the rear quarter panels, you can't make it look identical. Other than that, if you want it to look close, it's basically half a dozen emblems, an SS grill, rims/tires, and 1 1/2" lowering. ROH still makes the wheels that were used on the SS, and GM still makes all the emblems and the grill shell. You will also have to remove all of the Caprice's moldings, hood ornament, etc. Keep in mind this is exterior stuff only. If you want the interior to look like an SS you would basically have to get the entire interior from a wrecked Impala.