Jamie.
There's a lot of variables.
1/ Website Design.
Build / Set up a bare templated design CMS (Content Management Sytem - where the client adds all of the content, including extra pages) say, £ 250. There is nothing wrong with templates.
As above, doing the work. Including on page SEO (whatever that is nowadays?) say £ 500.
Unique graphic design and hand coding say, £ 1,000
Keyword research and silo building , hundreds upwards.
Professional copy writing minimum £ 1,000, maybe a lot more. Professional videos or photo's, same again.
On going website promotion, including tracking, testing and re targeting say, £ 150.00 - sky's the limit per month (depending on competition). Following the recent updates, this just got many times harder.
2/ Hosting
Maybe £5 - £ 10.00 per month although most people would include this free for the first year.
In conclusion, maybe a couple of hundred up to many thousands, depending on the amount of work needed. In the right situation, a cheap site could still pull in a large office job worth thousands.