There are a few ways to get a Drupal theme for your site. These can be quite easily placed along a continuum with "Low Cost/Difficult" at one end and "Higher Cost/Less Hassle" at the other end. It all depends on your time, skills, and budget whether you want to go more of the DIY route, hire a professional Drupal themer, or somewhere in between.
If you know XHTML/CSS and know your way around a graphics editor, you may very well be able to create your own Drupal theme. Knowing how Drupal’s bits and pieces work is more important than any kind of advanced PHP knowledge. The difficulty of this vastly depends on how complex your desired theme is and how your skills are. A Drupal theme can take anywhere from a day’s work to hundreds of hours. Most of the themes you see here on TopNotchThemes took experienced designers/themers 30-50 hours to create. Your first theme won’t be your best theme, so make sure this is an appropriate decision if you decide to go the DIY route. For your first time, you’re probably best off trying to modify something already out there.
There are tons of free and a few very low cost themes out there. Most are available on Drupal.org. These vary in quality and the nicest ones usually get widely used on many sites. If exclusivity of your theme isn’t a big concern, these are a great place to look. One of the most common approaches is to take a free theme and customize it by adding a different header or changing some styles in order to get a more custom look. This is a great approach if you’re new to Drupal theming. It will help you get an understanding of how things work but with a good structure beneath you. These often have a "stock Drupal" kind of look to them so you’ll have trouble finding something completely unique.
If you’re on a tight budget but want something customized and don’t have the skills or time yourself, you can also hire a Drupal themer to spend just a few hours customizing an existing theme to suit your needs.
This is really a new category in the Drupal theme world that we like to think we had something to do with ;) A premium theme is one that has a cost justified by the design and coding work, and support that goes into providing the themes. These will often vary greatly from that "stock Drupal" kind of look, and are much more limited in their distribution. A premium theme isn’t for everyone, if you’re doing an extremely low budget project or need a custom design, but they are a viable option for what’s in between.
A custom theme involves having a design created for your site from scratch, usually by a designer who then provides mockups (or mockups + CSS) to a Drupal themer. Trying to pin down the cost of an "average" custom theme is kind of like asking how much a house cost — it really depends. Bare minimum for an extremely basic, single page design would be a few hundred dollars. Average cost for a small business site design + Drupal theme would be in the low thousands. This can vary widely with things like number of unique pages, Drupal module support required, if you need a logo, etc., but hopefully that gives you a general idea. It’s worth noting that there is a shortage of good Drupal themers available right now — demand is exceeding supply!
The right choice for your web site project’s theme should fall into one of the categories above. Make sure you carefully weigh your timeline, risk, budget, and skills before deciding on one of these options. In our previous experience doing theming as part of a Drupal project team, the theme is usually 25-40% of the budget for the entire project. Drupal’s themes aren’t just a pretty skin on a functioning website — a lot of functionality is exposed through the theme itself and it can be an uphill battle if you don’t know what you’re doing.