Premium HTML/CSS themes are not that expensive and usually much cheaper than themes which are already developed for a content management system like Wordpress. You can get good looking premium themes with a standard license beginning at a price range of around 10 to 20 dollars.
Also there are many free HTML/CSS themes available on the web which are perfectly fine to be used for your web project. Often they require you to keep a link on your page back to the theme creator to give credit. Removing this is also often possible for a small fee.
Differences between free and premium HTML/CSS are of course existing, but depending on which themes you compare the differences might not be that big. Especially looking at the scope of how big a web project might be, using a free HTML/CSS theme can be a perfect solution to start with. Even if a project needs to be converted to use Wordpress at some point it is of course possible to integrate wordpress into your existing HTML theme without changing the layout or design.
free HTML/CSS theme can be a perfect solution