Mike,
I never, ever wash "just the dirty parts" of a house. It really can't be done since all surfaces get dirty, some just show it more. When you clean one side, the cleaning solution you use will likely creep around the corner and you will then be trying to blend in the dirt. Its not worth the effort.
Having said that, you need to establish a minimum price. $75 to $100 is not unreasonable, assuming you are licensed and insured.