Depands on the state you live in. You can do a DBA but I would set up an LLC for sure. On the cheap expect to spend approx. $500
If you're intending to operate your business as a sole proprietor or trader (for other countries), then your only start-up costs are usually those associated with acquiring a business license and establishing your business name. Partnerships generally work this way as well, with only minimal differences in paperwork and needed filings. Other structured such as a LLC (or Ltd) or corporation would likely require an accountant or a business attorney. I've seen people start a business for as little as $25, and then there are others whose costs were well into the thousands. Of course, this is all dependent upon the business structure you choose and whether you hire a seasoned professional to do it for you.