Hi ctsooner, there is no "average" monthly cost for keeping a system fully powered up 24/7. It depends entirely on the current draw. In the late 1990s I kept my old Rowland M7 high bias 350W monoblocks running at full power for some three weeks in Austin (TX). The overage on my AC bill came to about $15. These days I keep my system at full power all the time, including the Rowland M925 430W class D monos, and my montly electric bill is minimal... Wife never complains.
But leaving the system at full power is not purely a matter of minimizing system stress during rapid thermal excursions.... I have experienced with my gear over the years that it takes a couple of hours to up to a couple days for a system to reach peak performance... That is running at full power, and grinding some audio signal. Even if fully powered up, the first CD I play after a few days of inactivity ends up serving as "warm up".... The second CD, is quite nice, and by the third one the system is glowing with music.
G.
G.