hi unfortunatley it would be measure out jobbie and add approx 10% for wastage and different depths in the building and garage. I would say measure in total and forget the windows and doors that should cover your wastage.
then just deduct the garage door out of this.
If you are going to paint you can use std gey cement which will work out cheaper for you rather than going for the white snow crete. I would try and get blue cicle as that is about the best.
Once you have your meterage worked outt this should be able to give you an idea on what sort of price your paying for the job to be done. I normally ask the customer to get materials as i dont have the funds to supply as well, I would get a labour only quote and one with materials included. Price up the stuff yourself and you will be able to tell who is trying to get the most out of you.
Also i would go for somebody who has been reccomended and you can go and see the work they have done, there are a lot of dodgy people out there for a quick buck and could leave you well out of pocket.
As if they have a portfolio of their work as well. The good guys have this, im working on one as i go along but forget to take the camera half the time lol.
If you can measure out and get back to me i should be able to tell you roughly what you need materials wise
Cheers
Paul