It always depends on the size of the building, type of the building and type of cleaning services that are provided. If in the beginning it is difficult to know what to charge per sq.ft. So my advice to you is think how much time you need to clean the building. Let's say you have to clean 10000 sq ft office on yourself and you need to clean it 5 days per week. And based on your experience you know that you need 5 hours every night in order to clean it. Also you know that you charge your clients let's say 17 $ per hour.
Simple calculations show:
17 $ per hour x 5 hour per night = $ 85 (that's how much you charge for cleaning per night)
You have to clean it 5 nights per week, so to know monthly cost you calculate:
$ 85 per night x 5 days per week x 52 weeks per year / 12 month per year = $ 1841.67 per month
Now it's easy, just take your monthly cost and divide it by 10000 sq ft :
$ 1841.67 / 10000 sq ft = 0.18
In this case you charge 18 cents per sq. ft.
In future you can do the same math for other buildings you clean and you will come up with average number
Remember, the bigger the building, the smaller the number and wise versa.
Also difficulty matters a lot.
Hope this helps,