There are different appraisals: those for insurance purposes (e.g. replacement value: loss and damages), and those for retail and trade-in value.
A rug appraisal should be done by a reputable organization whose appraiser is a current member of Orra (Oriental Rug Retailers of America) and is a CRA (Certified Rug Appraiser - click here for a list of appraisers near you). A Certified Appraisal will run in vicinity of ~$250 - 325 depending on location and specifics.
A CRA should always be produced on fine letter-head of the appraising organization. The appraisal should include a photograph of the actual carpet either on or attached to it. The appraisal will specify the rug's origin, the approximate weaving date, any rare attributes or unusual design features, and notes on the rug's condition including repairs or other distinguishable markings which may or may not affect value. Finally, the appraisal will contain a suggested value range with signature and date of the CRA. A thorough appraisal may require research by the CRA. If the rug has a signature panel or cartouche with inscriptions, it will not necessarily be deciphered on the appraisal. This most likely would be an additional cost which of which the request for such information should be clarified prior to inspection.
A Certified Appraisal should not be:
Entirely hand written
A "certificate" the seller has used for advertising purposes
A suggested value written on paper which includes other transactions made with the company such as cleaning, repairs or original purchase of item.