This is a real "piece of string" question. There are many factors that influence the cost of building a website, and the fact that the project is built using Umbraco makes little difference.
The key factors that we aim to understand during the early Discovery phase of a project are the size and complexity of the proposed website. The complexity of functionality and volume of work have the greatest influence on project cost.
The key questions we use to understand this complexity factor are:
How many types of pages will be in the final site; and
How complex is the functionality of these types of pages?
In this context when we talk about the "types of pages", or Content Types as we call them during IA, we don't necessarily mean Design Templates or even Umbraco Document Types. We mean more generically a grouping of pages in the site that will have a common look and feel, set of functionality and business rules.
In reality, there is a strong relationship between the Content Types we identify during IA and the resulting design templates produced by our designers and the Document Types created by our Developers. But they don't always exactly match.
The total number of pages in the site is not always a good measure of the final cost of an Umbraco site. Of course, someone has to write and edit all the content on the site, and this has a cost. However, when considering the build costs of an Umbraco site, whether there are 10 pages or a 1000 pages of the same Content Type makes little difference to the designers or developers. If they are designing a template and writing code for a single Content Type then adding more pages doesn't add substantial effort.
Other important factors that can impact the cost of an Umbraco site include:
Use of paid for Umbraco components;
The level of different "non-functional" requirements the site needs to have baked in such as reliability, accessibility, performance, and security that the site needs to achieve;
How fast you need the site built;
The day rate of the Designers and Developers working on the project;
All of this means depending on many different factors an Umbraco site could count anything from thousands to hundreds of thousands of Pounds.
If you'd like us to give you an estimate of the rough costs of developing your next project then contact us.