Some fans cost twice as much as others because they have motor bearings which are bathed in oil. Such fans typically carry lifetime motor warranties. Of course, if you plan to operate your fan only on a half-dozen hot summer nights, you won't wear out the less expensive, oilless fan in your lifetime, so save your money.
HANGING ON THE CEILINGS
If the fan is to be mounted on a sloped ceiling, make sure the fan mount will accommodate the slope. You don't want the blades running into the ceiling. Most fans come with a ball-and-socket mount that allows the fan to hang like a pendulum. The range of slope accommodated, however, is generally quite small -- typically only up to five in 12, or about 22 degrees.
If your ceiling has a greater slope, buy a fan with a greater range or one for which a high-slope hanger accessory is available.
Of course you can also attach a wood or metal wedge to the ceiling to provide a flat mounting surface that will bring the blades down far enough that they won't touch the ceiling.
Fans can range in price from approximately $50 to several hundred dollars each.
Depending on what your needs are, you could easily spend several thousand dollars outfitting your home with ceiling fans.