The most important considerations in a stereo amplifier purchase are power output, sound quality, and price. Amplifiers range in power from about 10 watts to 1,000,000 watts. If you're looking to power a small bookshelf speaker, you don't need anything more than about 30 watts. If you want to fill a small room with sound for parties, then you'll need about 75 watts. The more you pay for an amplifier, generally, the better sound quality and the greater power output it will have. The simplest, and cheapest, amplifiers can be purchased for under $25. The most expensive, and best-sounding amplifiers, can cost over $2,000.