Two things,one ladder.
What it costs to build and what it costs to keep running. A site with a backend is not a different discipline from a site without one — it is a step further up the same ladder, so it lives on the same page and in the same price list.
Not sure which one you need?
No sales pressure and no generic proposal. Tell us what you are trying to build and you will get a straight technical read on it and a clear next step.