A little late, but....
If you are utilizing all electrical components for both the Pump and Air Compressor, the average cost is about $0.78 per hour. To break it down for you, both the 6gpm and 7.5gpm High Volume Pressure Washers require approximately 5.29kWh. The air compressor requires 1.8kWh. Figuring the average cost per kWh in the US as of January 2011 was 11 cents per kWh we have the following equation: (5.29kWh + 1.8kWh) x $0.11 = $0.78 per hour. If you know what your current electrical costs are you can plug it into the same equation for a more personal estimate.
For the water, figure 2-3 gallons per minute. 3 gallons x $.0015 = $.0045. Factor in your own water costs if you have them.
So for eight hours, $.78 + $.0045 = $.7845 x 8 = ~$6.27
So, buy the system and give your parents a little change every time you run it.