The Brake Booster doesn't hold any fluid whatsoever. The seal on the rear of the Master Cylinder goes bad, and can leak into the Booster. The problem with that is, if the Booster fills up with enough brake fluid, you need to replace both, as the fluid will damage the seals inside the Booster.
Chances are, you just need the Master Cylinder. It's rare that the Booster gets damaged, I just don't want you to have any surprises.
$800 seems a bit steep for Riviera brakes.....quite steep actually. Given that he's not the dealership, your mechanic is probably getting aftermarket parts, which can be had by him quite cheaply. Even at $50 for the pads, and $50/rotor, you're only lookin at $300 for parts. Let's say $400 with markup. Another $400 for labor when EVERYTHING is being replaced is a bit ridiculous.....just sayin.