A few years ago, my wife and I took a Pontiac Grand Am to M&M because it had a cracked block.
The initial estimate was $200. We got a call about 2 days later saying that it was going to be $800, and they'd already started the work.
We said no, because the car still run. They said "not anymore: we've started taking it apart". They offered to keep to their $200 estimate if we just towed the unworking car away, and they'd give us the parts they removed in a plastic sack.
They really had us over a barrel... we had no choice but to pay the $800 (even though the car was only worth about $1500), but we'll NEVER go back there again!