basically, figure out what the absolute minimum you need to do the project, then add on top what you think the client is willing to pay.
A friend of mine is charging something like 22k for a non dynamic site that will take about a week and a half (for a big advertising agency though that tends to throw money about) - on the flipside they are also doing projects that take just as long for about 800 quid.
we're doing a site now for about 5k when we should really be charging 12k or thereabouts, but we know it will lead to more work. It's swings and roundabouts really ...