There are really two factors to consider.
First is transient voltage surge suppression (TVSS). TVSS prevents dangerous overvoltages that result from lightning strikes or faults in the distribution system. They keep your equipment from becoming toast. Any unit you buy for critical applications should be listed under UL 1449. Many bargain TVSS units are UL listed, but are listed as power taps and not as a TVSS. Good units are $50 or more, rack mounted units with very low clamping voltages and energy let through are several hundred dollars.
Second are power conditioners, chiefly voltage regulators. These units typically incorporate a TVSS with an electronic system that keeps the output voltage steady at 120V RMS. In addition to keeping your equipment safe from surges, power conditioners can extend the life of your equipment by preventing the stress that improper voltage can cause. Most units also incorporate RF filtering to prevent noise from entering through the power system. These units are much more expensive than a basic TVSS, expect a good unit to cost >$1000, a basic model $>500.
Power conditioners for audio/video applications are different than many that are designed for computers. Good audio/video units use torroidal autoformers instead of iron core transformers, they also use electronic zero crossing switched taps on the autoformer. This type of design minimizes stray magnetic flux and prevents "switch noise" that can result from non-zero cross firing of taps.
