Power from an amplifier is dependent on three things: the gain of the input signal, the voltage at the output, and the power dissipation of the output devices. The more output devices there are (tubes or transistors) means the more power can be dissipated without them burning up. So if you want a tube amp that can deliver 40, 50 or a 100 watts of power, you need more tubes. And more tubes means more parts and labor and that drives up the cost.
