Oh, and how much you'll pay is highly variable. It depends on the job market where you are, and how saturated it is with competent web designers. A ballpark of $5K-$10K is not ridiculous, though, depending on the amount of work.

And what if it fails to impress potential users? Then your web designer wasn't very competent, because that's exactly the sort of thing he's supposed to understand about your audience. A good web designer will bounce ideas off of focus groups and do usability testing, and then when the site goes live will conduct A/B tests.

Finally, the best way to make sure that the site doesn't impress potential users is to micromanage the web designer. You need to ask him or her about the web analytics numbers, but if you insist on wordsmithing on every page and monkeying about with fonts and layout yourself, you might as well just do the site yourself. (This is something I've seen repeatedly: the web site winds up being designed for and by senior management against the advice of the web designer, the potential users stay away in droves, and the web designer is blamed.)