The short answer is not necessarily.
If you have already built your own website, you design, or code, you know this is a fundamentally easy question to answer. For everyone else its not.
The assumption is if I have 20 pages that will cost more then if I have 3. Although technically there will be a price increase it is not a proportional one. What you must keep in mind is that the cost is associated with each different page layout you have, not the number of actual pages.
Think of this analogy. You are fitting out the rooms of a new hotel with 70 rooms in it. Out of those 70 rooms 20 will have double beds and the exact same furniture and decoration. 50 rooms will be executive suites with kind size beds, desks, and again the same furniture and decoration. Now your best friend wants you to work on their boutique hotel with only 20 rooms, but every one of these rooms will be fitted out and decorated differently. Which hotel do you think will cost more.
Website building is the same as any manufacturing or repeated process. The more the infrastructure stays the same the less cost it is to you.
When discussing new site designs with graphic artists that I work with, I try to emphasise this exact point. You do have complete creative freedom, but keeping the layout of the pages similar or with only 2 or 3 variations, brings the development time and costs down. This has an added user benefit. The usability of the site goes up the less variation there is page to page. Your customers know where to find and expect things.
In the end, keep your page layouts to a minimum is a win win situation.