That seems a little excessive... when you say "unit" what are you referring to?

There's a cylinder front and rear, a central pump/valve assembly, and the lines in-between all of them. If your vehicle is as rusty as you say, it wouldn't surprise me if they've quoted every single bracket, bolt, retaining clip, etc. along with additional labor to Sawzall and torch things off. Still seems a bit high for all that, but I haven't priced out a KDSS system or looked into the labor for replacing the entire system.

One horribly rusty Sequoia I worked on needed a Sawzall and our oxy-acetylene torch to remove the old shocks, brake lines, and other bits (they spent way too much money on this thing when it should have been junked). Much of the brake system, power steering lines, and such including brackets were zip-tied to the frame or strapped because the frame had holes or was soft in spots.