i needed a cheap screen my self not to long ago...was gonna just get green fabric like most said...but didnt find green but got a very bright red...which worked good ..untill i opened my mouth and you can see right through the backround lol...so i found out the stupid way of why it should be green/blue lol...any ways the 100 % cheapest way i found after that was go to wal-mart where they got all the art crap (markers,crayons and such for kids) and look for the big poster boards they have there (just think paper)...and just grab as many neon green squares you need and your all set...just tape them side by side and start making your movie
they are only 89 cents each ,really i dont think you can get any cheaper and they are VERY bright so its great for chroma keying and such i have a 8ftX8ft wall in my house of it and i love it

edit...just say the OP asked that about a year ago lol...well hope this helps anyone else lol