........Purely in financial terms, that piece is worth monetarily what someone else is willing to pay for it(and that all depends on condition, type of frame, edition size, etc.). Sometimes its not worth more to anyone. Occasionally, one of my sold-out editions is resold at a gallery or by an individual at a higher price because there is a demand that may exceed a very limited supply. Also, galleries, because of their increased overhead and their interest in making a profit, will always charge more than if the piece is bought directly from an artist.
.......Galleries and art promotion companies have a huge motivation for fostering the concept that their represented artists' works increase in value both as a justification for their markup(they are middlemen) and for the sake of their future profits. They spend money and effort perpetuating a mystique surrounding artists and their work. Often their time and money is spent on mind-numbing "match-the-couch artwork" which none-the-less has a huge commercial appeal. This is marketting. Many fall victim to it-and many profit from it. If an artist tells you his work will be worth "x" amount in so many years he or she is treading on questionable ethical ground and you should be considering the source of motivation. Artists are not appraisers but there are many professional appraisers in your phone book and on the web who will gladly take your money.
......Usually, when someone asks me what a piece is worth, they aren't the ones who bought it originally. Chances are, when it was purchased, it had a message the artist wished to convey and it spoke to the buyer. One really does a disservice to the artwork, the painter, and the original buyer by reducing this to a "how much can I profit from this" kind of experience.