Holy shit.... not only did they overcharge you around... $200 dollars, but they completely misdiagnosed the problem. With a complete sink to the floor, I'd say master brake cylinder, but lack of fluid could also do it (I don't think it could be the boost lines with a complete sink to the floor).

Hell, I'd yell and scream at them. Well, not quite, but I'd make them take money off for so missing what could be a VERY DANGEROUS failure. If your master cylinder goes out while you're driving, like in an emergency stop situation on the freeway, you could get into a serious accident.

Though, before I continue... check the brake booster. With the car completely off, pump the brake until it's completely firm (5 pumps). Then, with pressure on the brake, start the car. The pedal should drop an inch or so. If it doesn't drop or goes all the way down, the booster could be at fault too.

Anyways, what you should do is call around to a couple dealership-alternative shops in the area, and get price quotes on a master cylinder replacement. Should run about $300 with all new OE parts. (~200 for the part, plus hour&half labor to replace it and flush the system). Find a good reputable shop that isn't a dealership, and go there. Avoid dealership repair unless your car is under warranty...