To give you a bit of an idea: at Eurocade 2011 we had about 40 games running, 6 of those pinballs, 1 large Sega (projection) Star Wars Arcade, 1 SW cockpit and 3 cocktails.
The games had been running from about 9.30 AM till 1.30 PM with a break of about an hour for dinner.
Cost us about 23 Euro's to pay the electricity bill....and energy prices are HIGH here. So...it's not that much really.
Most manuals and sometimes even flyers specify the number of Watts a game uses. However, these are average values/estimates. Let's assume a regular upright cab consumes 120 Watts. If you run that for 10 hours it would be 0,12 kW x 10 = 1,2 kWh. Multiply that with way 1 kWh costs from your electricity provider and you know what it costs.
(over here it is about 0,22 Euro per kWh, so it would be 0,22 x 1,2 kWh = € 0,264, which is about 0,33 USD.