There are several benefits to an attic exhaust fan.

When purchasing an attic fan, size is the most important factor.  You want a fan that is big enough to make sure the attic is cooled, but not so big that it wastes energy.  Size is a function of cubic feet per minute, or CFMs.  To calculate CFMs, measure the square footage of the attic and multiply it by 0.7. 

Standard fans are sized at 4,500 to 6,900 CFMs.  These are the most common and most affordable fans.  While they do cost less initially, installation may be more difficult…and potentially even more expensive than the fan itself.  For example, you may have to move a joist in order to frame out a box for the fan, or you may need to install additional attic venting.  Also, during the winter, the vented opening work gives warm, most air an easy path out of the attic.  Preventing heat loss may require an insulated box to cover the fan