Bitcoin’s value is dependant on the intensity of demand and supply there is. The demand is a market factor. The intensity with which the market needs the commodity fuels the demand aspect. Supply however is a limited factor within the Bitcoin economy since the number of bitcoins there can exist within the network is limited to 21 Million. This often causes a difference between the buying and the selling price. To answer your question, there isn’t any actual set price for a Bitcoin. The price of Bitcoin is a balance between what price someone is willing to trade with to part from the Bitcoin and what someone is ready to pay to acquire it.
Bitcoin exchanges and companies that transact using bitcoins determine a lower and a upper limit around what a person in possession of Bitcoin is willing to sell it for and what a person is willing to pay in order to acquire it.
A price is determined after setting upper and lower limits between a seller and a buyer. A price is determined between the limits based on a pricing strategy that the company(exchange) employs.