Yes, I'd say windows are profitable! From everything I've read, a good rate is around $2.50 per window per side downstairs, and a bit more for upstairs windows.
My first job on my own was to do the windows for a homeowner and pressurewash a small patio. It took around 3 1/2 hours, and paid $225.00, most of which was the windows. Not bad for something that has literally NO overhead. $10.00 in cleaning chemicals will last you for a LOT of windows, and the squeegees don't wear out that fast and it is very cheap to replace the rubber when they do. A guy could literally get into window washing on a $30.00 budget. A couple squeegees, some good lint free rags, some soap and water, and a good polishing cloth and you're in business!
I agree with Beth, it is a great add-on, especially for someone like you who is out at night at commercial properties anyway. Get them to pay you to do their windows while you're there!