Could be competitive.
Book time is 3.2 Hours. Average private shop rate is usually $50-$75 = ~$200 in Labor
Here's the deal with parts. If the shop buys the parts from a local parts store, and installs them, and something fails, the shop will (almost always) do the legwork and the parts store will cover the parts and labor cost (meaning you bring them the car, they fix it, and charge you nothing). The shop will mark up the parts on the original purchase for that service.
Decent loaded rear calipers are probably about $100ea over the counter at the store, so figure $150ish through the shop (these are pretty loose numbers and dependent on individual shops/suppliers, but probably within $30 or so). So you're touching $300 in calipers. Depending on how extensive their inspection was, they might be guessing that the calipers will be seized, and it's completely possible that once they get in there, they might find that the old ones are fine.
$150 for pads, rotors, brake fluid, maybe shop supplies. That may or may not include tax....not completely unreasonable.
Keep in mind, when you buy parts, you're buying the warranty. You can always find a cheaper source, but they might not stand behind the part. You could buy it down the street yourself and bring it to the shop, but they're not going to replace them for free if the part fails, and the parts store isn't going to cover labor on an over-the-counter sale, and you'll probably have to re-buy the replacement part and return the defective part for a refund.
If you're prepared to take that risk/hassle, you can certainly save a bunch of money, even if you have the same shop do the labor.