Asking "How much does a website cost?" is a bit like asking "How much does a car cost?", and of course the answer is "it varies, how much are you looking to spend? What do you want of the car/website?". Surveying various designers under the pretense of addressing this question is price fixing.

The fact that the designers were asked their prices on the criteria that "All prices are for design only, i.e. they don’t include any sort of coding (no HTML, no CSS, no Javascript, no PHP, etc.)." is rather telling with regards to experience.

Design doesn't stop at the mockup stage — anyone who has designed & built the front-end of a website knows that the design continues to evolve outside of Photoshop. The nature of the web dictates a website must be flexible so it can accommodate different browsers and resolutions. It's very difficult (read: expensive) to design for a dynamic medium in a static work space, and it's disingenuous to call a static, non-interactive image a website.

This post would be better titled "How much can one charge for a website mockup?"