Supply
In economics, supply refers to the market's capacity to produce or provide commodities, and demand refers to how desired the commodity is in the market. When supply is larger than demand, the price of a commodity decreases. While when demand is higher than supply, prices will increase.
Theoretically, if we said that supply and demand have met, then the price of a commodity is its value. However this never happens.
Issues
Manipulation
Supply and demand are always easily manipulated by capitalists, via creating artificial scarcity.
Relation to profit motive
Due to the profit motive and capitalist manipulation of the market, a commodity's price will always exceed its value.