Alot! Most of it is time, and time is money. It can be very difficult to account for the costs, since they are not tangible objects.
First, you need a name - so you’ll have to spend the time to choose one and then register it.
Then, you need a server, for the site and for email (it may be the same server), a server is the computer that delivers the site. You can put one in your office, choose a hosting company, or let a company that builds your site handle the hosting. Each has its own costs and benefits. They are offered in decreasing levels of skill requirement, but increasing cost. This is a recurring cost, it will affect the reliability of the site and the cost of maintenance. It also affects the level of control you have over your site and system. You also need an Internet connection - and a high-speed connection to support a busy server may be very expensive.
The site has three main components, design, content, and logic. They can be provided by different organizations. If one company has many sites with designs you like - get an estimate from them for a design. Whether you purchase just a visual image or the HTML/CSS code must be specified. Content is often best managed by an application - either a blog or content management system (CMS). That can be done in house, with some training, or outsourced. The logic may be the blog or CMS. Don’t build one - buy it, or use an open source solution. Visit some sites and run through the demos, see if the person who will manage the content likes the applications as well. Integrating the application and the design should be done by a company with experience with that product. This applies to ecommerce solutions as well.
Be prepared to clearly state what you are trying to do. Try to stay focussed on that objective throughout the project, changes can be very expensive, for all parties. If you don’t have a clear objective, it may be better to wait until you do.