Carey, most of the guys that I know of that do fleets and want the spot free rinse use the DI systems. Chris T. is the first one that I heard of that uses RO for the spot free wash.
I see what you mean about the cost, I was looking and it seems to be between $0.20 and $0.25 per gallon to make the DI water with the system. RO would be a lot less expensive but I do not know that much about it and how much water it can make (gpm's) or how long the filters or anything else that you change out would last.
My best advice is to keep searching out there and maybe on the Window cleaner's bbs, they have some good systems for DI, maybe they have something for RO also?
Let us know what you find out.