Projector screens can cost a lot - but they don't have too. There are some great products that let you make your own via a kit or you can build a screen from scratch.
(no a white sheet doesn't count) Projector Central has spent some time and compiled some great info for the geeky do-it-your-selfer. They did have some strict criteria that the screen had to follow though. It had to be less then a $100 and be easy to build. Also, it had to be just as easy to install but most importantly, it had to preform.
How did they do? Well the total bill came to $98.50 and from the comparison pictures, the quality seems fine to us. (the left one is the home made one)