To put it simply, the price is chosen to allow the biggest number of transactions or, alternatively, the biggest turnover at any given moment.<p>Example:<p>Price    Buy        Sell<p>124 €    130 pcs    50 pcs<p>125 €    120 pcs    90 pcs<p>126 €    70 pcs     90 pcs<p>127 €    30 pcs     100 pcs<p>In this case, the price would be 125 €. 90 transactions can happen at this price; there is no other price which allows more transactions to happen because of the supply and demand constraints.