Allow me to clarify a couple things based in physics and fact not in marketing hype.
The engine, drivetrain, aerodynamics and load impact MPG, not the exhaust.
The engine burning fuel/air mix is the power source, not the exhaust.
Many exhausts will alter the sound, the trick is to find one that makes the sound you like.
Affordable is based on your wallet size. To some people $100 is affordable, to others $100 is a stretch.
Sure you'll find those that cite race cars open exhaust as power adds......but the exhaust is open to allow the engine to pump as much thru the pipe to sustain the high engine speed. If you want to boost "power" from your exhaust, you might try going to the next size pipe. Remember, to begin increasing size at the exhaust manifold and ports.
Suggest to consider Flowmaster, MagnaFlow, Hooker........
Good Luck.