If you're just "water sweeping" to get rid of mud, dust, etc.; between $.05 - $.07 sq. ft. should work. You'll have to get a general idea of average sq. ft. per house to offer a "set" price. Garages and sidewalks should be close in size from one to another. I would put a provision in for driveways to charge "up to X amount sq. ft." & charge for anything over that. You should not need ANY chemicals, and cold water should be enough. If there are oil spots or other issues, you need to make clear there will be an upcharge.
The downside of this work? It's a lot of repeat set up & travel. Most houses aren't done all at the same time, even if it's the same builder in the same development. Don't believe them when they tell you you'll be able to hook up and go from house to house on the same day. Not gonna' happen! The other issue to clarify is the expectations. Explain CLEARLY what is going to be done, and what area. If you don't, you're going to end up going back to reclean, doing the patios for free, washing down the brick face, etc., etc., etc.
The other big issue is resoiling of the area by other contractors. Always put in a provision that there needs to be inspection of the work when completed, and that any resoiling is the responsibility of the General or sub-contractor.
Biggest issue: Get Paid! Since you are on the back end of the building process, you're usually close to the period of the final "draw" from the bank. Set your terms explicitly BEFORE you get in too deep. Most builders are OK with paying, but they'll string out a little guy forever if they think they can get away with it.
i've been one of those strung out little guys as you say. took one company 3 months to pay me! you could technically measure every inch of concrete you have to do, divide it by the number of homes and ta da! you have an average per house cost. quite painless really.