How much should I charge for doing a site?

[0dev]

Because I'm the resident computer guy and I've set up a few websites before, I'm helping a business run by some friends of mine by doing them a website. Since they're friends I will be charging them less than a normal web developer would but I want to get a reference point for pricing beforehand.

What I'll be doing is setting up a shop with a CMS, doing a custom theme and a custom banner to their specifications, adding the inventory to the shop database (with descriptions, pictures, etc.), and setting up credit card payments.

If you were doing that job as a web developer how much would you charge?