Let me tell you as a website designer that there are lots of truly amazing themes on ThemeForest otherwise known as Envato.
They basically sell everything - website themes/templates, audio, video templates, images and more.
For $60 you can get a truly amazing theme for Wordpress or html/css templates for around half that maybe even less. I almost always buy a template and then edit it to fit what my clients want.
Laying the ground work ie the base of a wordpress template is ALOT of time and work. I can pass big savings onto my clients buying a theme and turning that into what they want rather than doing it all by myself. Of course if for a specific reason I needed to build it all myself I can but this costs me lots more time so them lots more money.
I wouln't however recommend buying a adsense theme as this will just be a theme where ads are in places where people are most likely to look and click. That is if you have a good theme developer who has actually researched this. It could just be any other theme with adsense built into it. You have no real way of telling if it really will make a difference. Also you could just mimic anything bits of the theme you like if you are able to write in html, php and css.
Just my 2 cents coming from experience. Main point is buying themes save massive amounts of time for a relatively small outlay.