When we did some Wordpress work it would vary anywhere from $2500 on up. Custom themes are always more time (e.g. more expensive). I'd say there is almost no website you will reproduce within 40 hours unless it is the most basic site containing only a small amount of pages.
Remember that clients will say, oh I want it to be just like X site, then as you work it they will start saying well no here I want it to do Y not X. And then they will cause content delays etc. And the smaller the client the more changes and indecision, in general, so don't flat rate it. Instead make sure to do it by the hour and tell them there is a minimum cost.
