How much would it cost to change my 1996 pontiac gtp automatic into a manual transmission...bc i have to replace my tranny bc it blew.. so i was just wondering if it is possible and hoe much it would cost me instead of just replacing the AT

The parts and fabrication required would make this swap very impractical if not virtually impossible. The 91 is OBD-1, the 96 is OBD-II, so you can't switch the computer/harness/etc... from the stick shift donor car to yours (to ensure sensor compatibility). The 96 wasn't offered with a stick, so no parts are available to make this swap work. Get another automatic and be done with it.