msgsobe,
I don't know if this is your first aircraft mx visit or not, but you need to understand that any quote on any type of aircraft is just for the inspection. This means you are paying for the opening, inspection, and closing of all the areas to be checked. Anything that is found after opening these areas that need to be addressed or parts that need to be changed, is an expense over and above the quoted inspection price. For our aircraft we always expect to pay about 3 times the initial quote (yet always making sure we monitor all the cost and get it done for the least amount possible) when we advise the owner of the aircraft about the upcoming mx. So given that - $250-300K is not out of line. Of course you should alway have a LearJet qualified maintenance rep overseeing the mx while it is there to oversee the work done and to watch the billing to make sure it is accurate (billed work hours vs actual hours worked, and to make sure you get credit for anything that should be covered under warranty - MSP and any others you may have).
Regarding held "hostage", I have heard of operators/owners, make this statement. But in reality they are just uninformed on how mx works and they get sticker shock and don't want to pay the bill.
Good luck!
J