Even more generally, this applies to commodity markets. Price of potatoes is x EUR/kg, set by supply and demand. If some farmer can produce potatoes for 0.1x EU

Even more generally, this applies to commodity markets. Price of potatoes is x EUR/kg, set by supply and demand. If some farmer can produce potatoes for 0.1x EUR/kg, they get to make a good profit. Now electricity wholesale markets are an artificial construct, but it has been designed to mimic other commodity markets in that the producer on the margin sets the price.