The short answer is: it varies. 🙂
Fortunately this post by Brian Krogsgard offers some guidelines to help you ballpark what you should expect to pay.
There is some truth in that you get what you pay for, but of course there are other costs, such as agency overhead, that will tip the scales a bit. Using a freelancer might offer a lower-cost option, but you may also have a higher risk of not getting what you paid for.
Each option has its own pros and cons and the post does a good job at covering them, as well as going into detail on types of websites (and associated costs), when to choose an agency versus a freelancer, and average rates.
This is well worth a read if you’re in the market to hire someone to build a website for you, or if you’re on the other side and need some guidance on how to price a project for a client.