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You have my sympathies, I went through this horrible and painful experience 2 years ago (shortly after I'd bought the car and just outside warranty). I wish I'd known about the plug to disconnect, as the high pitched noise that is constantly on was driving me mad - I had to live with this for several months before I found a solution. I'd been given a similar quote by Lexus of around over £2000 to replace the master cylinder booster jobby.
After a few months, I came across Dial4Parts who were able to sell me a used Master Cylinder part for around £500 (can't remember the exact price), I then got Lexus to fit it at a cost around £140 (2 hours labour!). Its 2 years on and touchwood all is well.
I've had a number of PMs from people on this matter too. This appears to be quite a common problem and an expensive problem. I don't know product recalls work, but just going by the amount of people experiencing this, I would think this should be up there.
To me it all seems a bit fishy, the ABS unit is not a critical part of the car, the car brakes function perfectly well without it. So why the constant high-pitched noise?, surely the warning lights are sufficient to let you know there is a problem. It is a problem that can be fixed in due time. These are luxury cars and the last thing they should be doing is to be driving them insane with that high-pitched noise. It was a few months before I managed to get this fixed, in the meantime, I still had to live with it and transport family and friends around along with long motorway journeys (2-3 hours) even loud music could not drown this noise.