How am I supposed to have an estimate for the labor cost of curtain installation?

I’ve been planning on a curtain upgrade for my place lately and am therefore pretty concerned about all the expenses. Also, since I don’t have a lot of time, not even for taking the measurements, I’ve decided about getting the entire procedure done from the professionals. Apart from the actual curtain installation, I also want to have an estimate about the labor cost, i.e. how much do I expect to pay in this regard. Also, how is the labor money calculated? Does it depend on the amount of the curtains being installed or the number of rooms being treated? Please care to share your thoughts about this.