i left the car with them for the day and that way they could fit in in around the customers that get charged full price.. i paid $120 cash for the cutting and welding they did, but that's after i made the mock up and such. i had it all ready to roll for them. after getting it home i did the wiring..
i figure they charged me $60 and hour. that's a fair and expected price for someone who can do something you cannot / will not do.
if you muffler shop is quoting $400 for the entirety of the install, including wiring, it may not be a bad deal as they will have 4 - 6 labor hours and some materials in it when completed..
if the welding part were completed, i could do the remainder in about half the time as it took the first time as it would be a simple matter of "doing it" vs. figuring it out first.