Why would the labor be less to install new arms than just the lower ball joint? The lower BJ is one castle nut, one snap ring, press the old one out, press the new one in, reinstall the snap ring, and reinstall the knuckle with the castle nut and cotter pin.
To do the whole arm, you still have to remove the castle nut and cotter pin, but then you must relieve the tension from the torsion bars (not easy) then separate the torsion bars from the lower control arms (not an easy task) and finally, remove the lower control arm bolts (which aren't going to come out nicely, if they come out at all)....
IMHO, replacing the whole arm is way more work than it's worth. I've got 175,000 miles on mine, and they're on the third (maybe fourth?? Can't recall) set of lower ball joints. Yeah, the bushings look like crap, but they aren't loose or making any noise, so as far as I'm concerned, they're fine.