I have had different experience. From building anything from desks to 6ft by 6ff beds, I have found it is always cheaper and better quality to build than buy...If you want something solid!

A basic desk will be fine from Ikea or similar if you just want something that will do. If you want something that will stand up to weird demands (multimonitors, kids climbing on it, use as stage in Guitar Hero etc) you are better off making good friends with a local timber merchant and learning the ropes yourself.

Plus, it is a whole lot more rewarding:)