There are a lot of inexpensive screen alternatives. Yes a white wall is an option. Over at AVSforum they have a lot of discussions, including which paint works best. As a next step up some use a sheet of Formica or cloth. The lowest price screens run about the same as your proposed projector:
[ame="http://www.amazon.com/Elite-Screens-EzFrame-Projection-Screen/dp/B000N3TZS2/ref=tag_stp_st_edpp_url"]Amazon.com: Elite Screens EzFrame Fixed Frame Projection Screen, 16:9 Aspect Ratio - 100in. (Cine White): Electronics@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51sm13%2BQnTL.@@AMEPARAM@@51sm13%2BQnTL[/ame] -- 100 inch is $367