I was fortunate enough to attend a Stewart Screen Workshop event last year, and Don Stewart himself said that a good flat wall with something like three good coats of matt white paint will be comprarable to a matt white screen.
If you need some gain, then Goo paint or screen material as has already been suggested will be the best options.
I used white blackout cloth stretched and stapled over a wooden frame as my first screen (pics on my web-site), then when I upgraded to a dimmer projector, went for some 1.2 gain material - I bought it from
www.ellie.co.uk and stretched that over the frame like before. It was a little whiter and brighter, but I think it proved that the BO material is excellent value.
The wood and black-out cloth cost around £28 inclusing staples and screws, and the pukka screen material was around £100 on it's own. I think you can get some cheaper stuff elshwere now though, but I don't know how it compares.
Gary.