Getting a second opinion is a good idea. The lower ball joint is riveted to the arm, so you either have to replace the arm or drill out the rivets on the old ball joint and re-attach the new joint with proper hardware. $75 seems reasonable for the labor. The $140 part seems kind of expensive though. Might want to check with autozone or advance and see how much one costs if you can get it without the arm. If they are replacing the whole arm, both prices sound OK.
