One huge difference between a hack and a pro window cleaner is the amount of time it takes to do the cleaning. A pro is going to know how to "fan" a window to squeegee it in one big stroke. A pro is going to know when to use steel wool to get the tough stuff off, and how to use a scraper so as not to scratch the glass. And a pro is going to know pretty close to how long a job is going to take so he can bid it correctly.