As you realize, the cost will vary dramatically depending on the particular components that are in the system, especially power amplifiers and components that contain power amplifiers (receivers, integrated amplifiers, powered subs). Although the $30/month figure Elizabeth provided is in the right ballpark for your system.
I looked up the power consumption specs for your components, which are available for all of them other than the Squeezebox (which undoubtedly draws almost negligible power):
DACMagic: 5 watts
SLP-05: 25W standby; 75W operating
GFA-555: 72W idle; much more when playing music at high volume
AVR-600: 3W standby; 120W idle; much more when playing music at high volume
BDP-103: 0.5W standby; 35W operating
2W Sub: 10W idle; much more when playing music at high volume
Squeezebox: ~5W (my guess)
Using "idle" or "operating" values as applicable (although I don't know how optimal leaving the SLP-05 "operating" would be with respect to tube life), that all adds up to 322 watts, or 0.322 kilowatts. Multiplying by 730 hours in an average month results in 235 kilowatt-hours per month.
Multiply 235 by your cost per kW-hr to get $/month. The additional power consumption that occurs when you are playing music would add to that.
Regards,
-- Al