2 Answers

150 loads at 9 gallons per load = 1,350 gallons  
150 loads at 6 gallons per load = 900 gallons
The difference between 1,350 and 900 gallons is 450 gallons.  
450/1000  =  45%
45% of  75 cents is (.75 x .45) = 33.74 cents.
You save less than 35 cents by using the 6 gallon D/W over the 9 gallon D/W  
For some reason, the cost per 1000 gallons of water seems incredibly low.  Perhaps you meant $75.00? 

Not  enough to worry about, since the more efficient machine which cost a lot more money to purchase and at 75c a thousand gallons it will take years just to make up the big difference in initial cost.

The new Mercury filled imported  light bulbs are a good example.  The amount one saves in energy costs are not worth the extra cost of the bulbs when purchase and the hazardous material disposal cost, when the Mercury filled bulbs burn out or get broken.