I think it depends what people want from the site. If they just want a "one page wonder" then it's a one off lump sum. Don't forget to include hosting and maintenance fees, that way you still have money for any problems that occur!
I think you need to estimate how much time you will spend and work out a flat rate per hour. That's what I do anyway. Also, make yourself stick to the path and don't stray it's so easy to think oh if I did it this way it would look really cool. I've managed to waste so much time at work recently and it's just not worth it if they're not paying much!