I'm a Trad cleaner and I have one customer that was in some work that I took over, and he has his frames washed every time the windows are cleaned - three bed detached with nine windows (small porch which is windows only). for that he pays 18.50, which I'll admit could be a bit more, say 20.00, and it takes about forty to fifty minutes. Although it is a bit of a work-up, I do thnk it is a good idea, in fact, I now do all my own frames when I clean the windows (even though I don't get paid), and when I look at mine to next doors (which I don't do), you can tell the difference.