After reading Brent Bice’s great post about cheap themes, I think it is important to see the other side and talk about how, in some cases, they can be useful for your business as a small web design firm. It's clear that if you are a business owner and try to create your own page using one of those cheap themes, and you don't have experience with theming Drupal, you are setting yourself up for a big surprise. Check level ten's post about this.
On the other hand, if you are one man shop and/or you are working for a customer whit a really short budget, this themes can be a savior. Specially if you are artistically challenged like some of us. One thing is for sure, finding $45-$55 dollars Drupal themes is not so easy, there are not that many; sometimes is better to find a responsive site template and do the conversion. Here is my little grain of sand on what to keep in mind when doing so. Don't expect a detailed guide, your process will depend of your knowledge and your skills, work on your strengths when you try to accomplish this.