I was at an irrigation field day yesterday afternoon late for a couple of hours. Interesting numbers you offer.

Some things from my notes: 

• It costs $25 per hour to run the average 100-125 hp diesel engine. That pushes a typical 1000 gpm on a typical 1300' Georgia pivot.
• It costs $7 per hour to run the same gallons per minute (approx $0.07/kwh) with a 100 hp electric motor.
• Conversion costs from diesel to electric run about $14K, depending on the availability and distance of 3 phase service. NRCS has a program that a farmer can apply for to cost share 50% of a basic cost of this. They do not have any cost share money to cover any of the increased expenses that the utility company may pass to the farmer of extending 3 phase service lines.
• Sub surface drip is going in on some Georgia farms. Installation costs can run $1400 per acre, with a life expectancy of 15 years. Energy requirements are low. Water has to be much "cleaner", so a good filtration and flushing system are essential, and expensive. NRCS is taking applications (beginning Nov '08) to cost share 50%. There are lots of other pros and cons to sub surface drip, but it is an alternative worth investigating.
• Lots of tools coming online for soil moisture, weather, and crop use monitoring to improve irrigation scheduling, too. 

As for our costs to run pivots, I use a number of about $12 per acre inch for diesel, with a range of $9 to $16, and $6 for electric, with range of $5 to $8. These are numbers I have heard quoted, and varies depending on how many gallons are being pumped from how deep. Some systems can't get enough out of the ground and have to pump from the ground to a holding pond before pumping again to the pivot, sometimes pumping 2 gallons to get 1.5 on the crop.