It depends on a lot of factors, but it should boil down to two things:
1. The cost of your estimated time spent = Factor in time to gather requirements from the client + time to research themes (if you're going to implement a theme) + the time it will take you develop it (or outsource development) + the amount of back-and-forth you're willing to do on tweaks and changes (can end up being just as long as it took you to develop it) + time to gather or write content + time to train client if you're handing it off to them (otherwise factor in the time for you to maintain it yourself). I've found that more often than not you end up spending more time on these things than you anticipate.
2. How much is an effective website worth to THEM? For example, a doctor's office website is worth much more than a website for a pottery artist who works part time out of his garage. You should charge based on the return on investment that your client will get from the site.
I do websites full-time, and I would charge between $3k-$7k for a site like that for a company like that, and it would include some basic SEO and conversion optimization as well. For someone who doesn't do this full-time, you should probably charge about half that, since you wouldn't be able to provide the same level of service as a full-time shop or agency.
Remember, websites are a combination of marketing, information, and art. It's easy to slap together a WordPress theme and call it a day, it's not as easy to implement a website that converts customers, helps the company reach it's desired business goals, and makes more money for the business. Think about what type of website you are going to give them and then charge accordingly.