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