The parts for disc brakes on these cars are relatively inexpensive. At your average parts store, new rotors usually run between $50 and $75 a piece, calipers are around $15 each and pads range from $20-$40 depending on the type you buy. Keep in mind that I'm not including new wheel bearings, brake hoses or the possibility of needing a new master cylinder as well. To be honest, if he's going to to a thorough job on your brakes, $400 doesn't sound too bad. However, I bet it will end up being more once he gets into it. If there's one thing I can tell you about these cars, it's that it's always worse than it seems and you should go in expecting the worst.
If you are patient and willing to learn, buy a copy of the '72 service manual and look into performing maintenance yourself. Granted, brakes may not be the right place to start, but I'm sure there will be other projects that will pop up. Check with Ames Performance or Performance Years for a copy. Congrats on your purchase and good luck.