The most important element in determining values in practice is what actually obtains elsewhere in the market, for all goods and services and in all markets. Whatever it is you may be selling or buying, your benchmark for determining what is a fair price is the price of comparable products or services.
If you are buying or selling a 3 bedroom, 2 bathroom house, you feel satisfied with your settlement price if the value is roughly what similar homes have sold for in the neighbourhood in recent times. If you are opening a hairdressing business, you will price your services on the basis of what others in the profession are charging.