In a way it's nice the mechanic told you, but in a way it sucks because of that instilled fear. I've seen mechanics use it as a scare tactic. If there is a hole I'd start looking for a new subframe or replacing the car. But i wouldn't pawn the car off on someone else in that condition. And I certainly would ignore book value.
I'm all about how I feel with a car. But you also should consider the cost of a "new used" or new car vs a repair you may need to make or may not. A $1,500 repair vs a $17,000 hole in your wallet, might be worth it. And you can always tell the mechanic, look here is y