I had actually replied to Northstarcllc, but let me see if I can help. You have 20 6x10 rooms = 1,200 square feet. You state a waiting lobby, but not how large. Also, we need the dimensions of the small waiting rooms and bathrooms as well as the big office to begin to give you an idea for an accurate price. I normally bring a measuring wheel with me when I do walkthroughs so I can get accurate measurements that will help me when I sit down to do my bid. Also, I only charge by the square foot for building over 10,000 square feet that are being cleaned 5 days a week. Buildings smaller than that or less frequently are charged per hour. Another determining factor will be your area of the country. I am in the Northeast so that may make a difference.
As for the 3,500 square foot office 5 days per week. I must assume when you say basic cleaning, you mean trashing, dusting, bathrooms, sweep/mop hard surface floors and vacuuming. In order to determine this, again, you should try to break the building down by individual room size and the taks that need to be performed in each room. When you do this, you can than put a time to each task such as cleaning a toilet (maybe 2.5 mins), sink (2 mins), mopping a floor, etc and arrive at a time for cleaning that room. Then add all of the times together to arrive at a cleaning time for the building. For buildings you're doing 5 days a week, keep in mind there are certain tasks that won't need to be peformed every single day or will not take as long as if you were performing them once or twice a week. You can split the dusting up where you do certain areas each night, reducing your labor time and still having dusted the entire building by the end of the 5 days. Trash and bathrooms, most likely, will be performed everyday. Once you arrive at a time for cleaning the building each night, let's say 1.3 hours for arguments sake, you then multiply by your hourly rate which is determined by your labor costs, supplies, overhead, etc. Let's use $30/hr for example. $30/hr x 1.3 hrs per night 5x/week = $845. So your monthly charge would be $845 per month to this customer. Good luck!