you should even contemplate designing a site for someone for less than £100/150 + hosting and domain name costs.
A professional designer would charge a lot more.
You'll find that when people are paying for a site they tend to be a good deal more picky about it, and you'll need to be firm about the number of revisions you'll make after showing the client the final design. Otherwise you'll be making alterations until the cows come home - it's a trap many new designers fall into. Get a reasonably detailed brief, and show the client a sketch outline of the site structure before you begin.