The $2 per window is the standard rate, but don't be surprised if they charge just a little more than the number of windows you have. After all, you have to consider their travel expenses to get there and whether or not they'll have a hard time getting to some windows (consider it an extra labor cost and its a fair charge, you try getting on a ladder 30 feet up and reaching out on one foot to get the job done :P). Don't go with the cheapest estimate either, you really get what you pay for in that business.
Let me know how it goes after you get it done, though. I'm genuinely interested in hearing the details on how they did the job.
