Preparation of the substrate is essential for many reasons. The floor has to be flat, so it is nice to start that way. Floors do not become flat by accident! Without getting to technical, there is so much to consider, that this is best left to a seasoned professional contractor. There is a lot on the line here. for example: ALL manufacturers will back their product warranties ONLY if proper preparation has been performed, observed and documented.
This service is variable with every application, and most likely changes from room to room. Accurately estimating the cost of the substrate preparation before actually doing the work is challenging at best. It is very likely that discoveries will occur that were not able to be measured before the work commences.
That being said ... a professional contractor can still estimate the cost of substrate preparations to a higher degree of accuracy than a novice or DIY'er.
A quality finished product starts here, with the prep work.