How much is a stamp of Princess Diana in a white chifon evening dress worth?

Identify the exact stamp using a catalog. This could involve learning about perforations, watermarks and colors as well as condition. The catalog will provide a value. The value is what a collector could expect to pay for a stamp in fine/very fine condition. If selling, most cases you would be lucky to get 75% of the catalog, unless it is very valuable, then an auction would be worth looking into. Dealers would be able to take a look at the overall collection and make an offer. Find a local dealer throught the American Stamp Dealers' Association. The most common American catalog for identification is Scott's. Others are Stanley Gibbons, and Minkus.