As with most things in life, it depends. I can answer with another question: "How much does a car cost? Or even a small house?". The answer wouldn’t be too far off.
BUT… that being said, some of the usual costs are: design and build, content production (eg: images, text, video), domain name(s) and hosting. Depending on your skills, you can do it all by yourself, or have someone handle all or some of the pieces.
Let’s assume someone else will do it for you. Design, build and content can be done as a one-off by an agency or by one or more freelancers. As a ballpark figure, a custom starter website professionally done, can easily land somewhere between £2,500 and £10,000. From the initial meeting to publishing online, the entire process usually takes from 1-3 months to deliver. Domain names and hosting are (usually) packaged together costing less than £50 per year.
If you need a freelancer to get it done, I’m here for you.