First of all, you need to understand that the brand has no value. It's just a small piece of paper with a picture of something on it. Valuable it only makes the desire and the desire of another person to buy this copy to your collection, while paying certain amount of money. It is obvious that the value of brands not tied to the world economy and global trends.
To determine the value of brands famous in their circles philatelists created directories. The most famous to date is the Scott catalogs (U.S.), Michaels (very detailed, German). In specialized stores you can find Russian directories, but they have not received such prominence.
The prices in the catalogues appear after a careful study of the market of post stamps. They are Advisory in nature and does not guarantee that the mark will actually be bought at such a high, for example, price. The cheap stamps, mostly large-circulation is sold in kilograms or pounds.
Most valued brand in perfect condition, in the truest sense of the word. They should be burnt (not to have mail printing), with intact teeth, with fully preserved the adhesive on the back side, no scratches and wrinkles. Cancelled stamps have value only if produced in small quantities and almost never met in the post use. Here, the postmark with date, and best of all even save the envelope.
The cost of the stamps depends on the General trends in the world of philately. For example, now the brand Union, the Vatican, Africa are sold at prices far less than listed in the catalogs, since those countries is "not in fashion". However, things can change at any time.
Other ways of determining the value brands are the auction houses and some Internet resources. However, turning to such sources, you must first inquire about the reputation of these in order to avoid unpleasant situations in the future.