Complex sentence: It cost $399, or $599 with a 12 monitor and a Radio Shack tape recorder as datacassette storage; before it, the most expensive product Radio Shack sold was a $500 stereo.
Split into simpler sentences: It cost $399, or $599 with a 12 monitor and a Radio Shack tape recorder as datacassette storage. Before this, the most expensive product Radio Shack sold was a $500 stereo.