I custom built my own exhuast system from the 4inch dump pipe back into a 3.5 inch straight through into a magnaflow rear muffler. I chose to go mild steel as in total it would cost me $450 vs $750ish (including the muffler). The price difference i would personally say is not in the labour but the material.
Mild steel will last many years. My first car which was a 1994 toyota supra is still getting around with the same mild steel exhuast and that was almost 8 years ago, i bumped into the owner a few weeks ago.