Upper or lower?
Looking at the parts diagram, it looks like the lower ball joint isn't available seperately?? That would make the whole knuckle $164 each. The upper ball joints are $123 each. (OEM internet prices-your dealer will be higher). Plus labor and alignment, the price doesn't sound too bad. But, if the lower joint is available seperately, it should cost $30 each.
I wouldn't get an alignment after replacing the ball joints, only the tie rods.
Definetly get another opinion, they probably shouldn't be worn out yet (either ones), but I don't know what kind of miles those 58K are.