The good thing about working with a web designer who can custom build your layout is that the designer can (if you insist) provide the blueprint on how the layout was design. This can be useful later on when you want to do a redesign and that web designer is not available to help you. You can then take that blueprint to another web designer, and he can begin updating the layout fairly quickly. Time is money and the more time he needs to figure things out, the more it might cost you.
Choosing a pre-existing theme might be more cost effective and faster, but it can be difficult to reverse engineer that theme when it comes time for modification. It's not impossible of course to figure out how a layout was built. It's just that a new designer would have to spend some time figuring it out, which could add more to the cost.
What you can do is find a great pre-existing theme, hire a web designer to build something fairly similar to that, then make sure he gives you the blueprint for future modification. (the blueprint is all the html, css, php, etc. codes and the step-by-step instructions for how the theme was put together).
There's also nothing wrong with going with a premade theme too. Some people use the same theme for many years and then eventually switch to a completely new theme later on.