The basic components of paint are resin, a carrier, and pigment. The resin in paint is what makes it adhere, while the carrier makes it thin enough to apply. If you have chosen latex paint, the carrier is water; for oil/alkyd paint, it is linseed or soybean oil. Pigment is what gives paint its color; the dealer adds colored pigments to titanium oxide—white pigment—to achieve the shade you want.
In addition, paint has other inert ingredients, such as clay, that give it its level of sheen. Small amounts of other carriers contribute to how fast it dries and other desirable characteristics. All of the ingredients, plus their quality, determine the price, which usually translates into performance. For example, titanium oxide is what gives paint the ability to cover, so, the higher the titanium oxide content, the more expensive the paint. Likewise, the more odorless the carrier in oil/alkyd paints, the higher the price.