Are you any good at DIY?
You can make your own for around £30 if you build a frame from wood, then stretch and staple some 'blackout' cloth over it.
The cloth should be available from most curtain shops or similar (they will know what it is), and is genearly white cloth on one side, and an off-white vinyl the other. It's around unity gain, and it's size should be fine for a 7ft viewable 16:9 screen (or slightly larger). I've some pics on my web-site if you're interested.
You can replace the blackout cloth with screen fabric from places like
www.ellie.co.uk or DRH screens, Harknesshall etc, and although the material is more expensive than the BO cloth, it will give you some gain if you need it. It also comes in sizes that are suited for bigger screens.
HTH
Gary.