The real purpose of a CMA is to establish current estimated value of a similar feature. This is achieved by investigating both the features and characteristics currently listed most recently sold in the same area. This information is generally provided to the owner of the property to help you establish a fair market price; This information may also be given to you at buyer in order to make this an appropriate offer to the buyer.
Some real estate agents perform this service free, may also charge a fee for this information. It all depends on who is doing the CMA and also how detailed the information was provided.