Lynn always responds to this question with "How much does a blue suit cost?" What a simple way to get across the fact that a website can cost almost anything depending on what you want the site to do for you.
I often add that what you really need is a "good" website. Notice I didn't say a "great" website? Of course all we want expensive designer shoes to wear with that alluring couture suit, but that's not realistic and it's probably more than you need to spend to look really, really good.
A "good" website has all the right elements layered from the inside out, yet most people just assume that all it takes are some graphics and a kid, or template, to get it coded. No my friend, if you think that way, you are headed for trouble. At least 50% of our business is correcting or creating new websites for clients who once bragged about how cheap their website was. Now they are bragging about about how good their website is.