Normally, molding a head can be difficult, but this method is simple and easy. With the use of paper mache, or plaster strips, you can make a 2-part mold, and you can even do the entire process by yourself. And, as always, without spending too much money. It should take about 45 minutes to make the head mold, and then another half an hour to make the clay copy.
Here's a video tutorial you can follow, or watch as an introduction to the overall process, including both Part 1 and Part 2. The step-by-step Instructables are more detailed and have more advanced tips and recommendations.