I know you and I have had some heated debates on Wordpress hahah but, think of it this way…if you offer your client a completely customized, built from scratch WP theme then you can charge them a good amount just for that. One huge selling point of a higher price for that, is you can tell your client that no one else will have that theme, it’s completely original and owned by them.
Now, with that in mind you can start adding to the price with features depending on the client’s needs. Do they want a ecommerce store? Do they want a blog? Do they want to host and display videos? Maybe they want to set up a profile system for users on the website. That’s all stuff you can charge for with WP (or any CMS for that matter)
The beauty of WP (and again any CMS) is that you can make it as complex or as simple as you want. But as soon as you start making custom themes, plugins, widgets, that’s where you take it from a easy install to something worth some decent $$$
And to add, while WP is free, you can totally charge for installing and setting it up for your client. A lot of people don’t want to be bothered or don’t feel comfortable setting it up themselves, so that’s where you come.