There's no picture to look at. But here's a few answers to your questions.
You can only replace the outer ball joint (the one at the steering knuckle). When you replace the control amrs you get 2 new ball joints, although I believe that it's the outer ball joint that tends to fail.
You're correct about how cheap just a ball joint is compared to a new control arm. The problem is that you need the right type of press tools to put them in. Once you factor in the labor to do that (since most people do not have the press tools), it becomes cost effective to just replace the arms which is pretty easy to DIY.
1998 328i/4 Boston Green
X-Brace, Eibach Pro Kit/Bilstein Sport, M3 Rear Sway
BMW Traffic Pro - a/d/s P650.2 & 3.45is - JL Audio Stealthboxes[/size]
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