The first thing you need to do is find out what the market is in your area. I live in Sacramento Ca, where the rates start out at $25 per hr. BUT...Further south from here, down in in San Diago, you are lucky to get $15, for various reasons I wont get into right now.
Anyway....
I use 3 combined methods.....

(1) $25 pr. hr.
(2) $1 pr. window per side
(3) Risk,Step, Pain in the....factors.

Bottom line, I want / need to make at LEAST $25 per hour (you dont tell / volunteer your customers this info) and I have no problem asking this much because I am already "established", and not going to starve to death anyway. Then I count the windows. It would seem that 25 windows would run about $50.00 right ? Not always. Next I consider "risk" factor. I.E. Do I have to climb up anything or use a ladder ? Is there possible "breakage" involved ? (meaning, you will have to move MUCH slower to avoid breaking things) How many steps are involved ? Do you have to get the tops of the windows with a pole, and then go back and hit the bottoms, or can you do each window in one fell swoop ?
Okay, now the pain in the.....factor.

Can you do these windows anytime, or must you show up ONLY at a certain time / day etc ?
Are all the windows easily accessible, or do you have to move displays, take down signs, scrape tape etc ?
Is this likely to be a steady / regular customer or just a one time "service call" job ?
In conclusion.....
Count the windows at X-$ amount per window, per side.
Figure in all the afore mentioned "factors."
Keep in mind you need to make X-$ amount per hour.
Now add $$$'s onto, -OR- Take $$$'s off the final bid, accordingly.
That should do it !
One word of caution...It is SOooo easy to be tempted to "under-bid" when first starting out, but dont, I repeat, DONT do it or you WILL be sorry, and very soon !
Trust me on this, I have been doing this over 15 years. I'll help you out anyway I can. I know what its like. Feel free to e-mail me anytime, although it sometimes takes me awhile (a day or two) to get back to people.
Hope this helps.
Chris / Shiner Co.

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Chris-Shiner Window Co. Sacto Ca.