It would be hard to give you a $$$$ estimate. I live near Tampa FL. Gear changes here are $500 at a local shop specializing in this area.Keep your same rear end, you will get nowhere close to hurting it with the mods your suggesting. Exhaust is a very debated area. You need to keep a certain amount of back pressure to scavenge out the exhaust gasses. I helped my son install a header on his Honda, gut the cat, and replace the tubing to 2.25" from the cat back. It actually ran like crap until we put a Thrush oval muffler on to build some backpressure.A cat back dual exhaust using standard steel tubing with turbo mufflers costs $200-$250 installed at the local muffler shop here. I cannot justify buying chrome tips and a heavy stainless steel exhaust system to put under my car when I can use the extra money saved to buy a shift kit or pulleys. Go to these links and read to get some different opinions for your own knowledge before you spend a bunch of hard earned cash.
http://www.3.8mustang.com/forum/arch...x.php/f-6.html
http://www.3.8mustang.com/forum/arch...x.php/f-4.html