It is against the Code of Ethics of museums to place a value on an artifact or collection.  Also, know that it is unethical for appraisers to base their fees on a  percentage of the appraised value of an artifact or collection. There are three main appraisal organizations: American Society of Appraisers, Appraisers Association of America, and International Society of Appraisers. Members of these organizations generally work as individual  contractors. Fees are based on the scope of the individual project and  should be discussed directly with the appraiser.

The Internal Revenue Service has issued transitional guidance on the new definitions of qualified appraisal and appraisers at - Guidance Regarding Appraisal Requirements for Noncash Charitable Contributions

Additional information which might prove helpful located at - Art Appraisal Services, IRS.  This information includes links to forms and publications, as well as addiitonal resources.