Burlap or jute netting would be a cheap alternative. Check out RAWR'S thread on Hauntforum and click on the pictures he has on photobucket
http://www.hauntforum.com/showthread.php?t=19770.
This is from his 2009 haunt. :
He told me to just cut the burlap into strips and then dye the strips with various colors of RIT. Then you slip the strips onto pvc, wood, or rope across your ceiling and it would hang in the path of the visitors. After that you could probably run a large roll of burlap across the length of your hallway/room to completely cover the ceiling. You would have to watch out though because it could catch fire fairly easily. Not sure if that's what you're looking for, but it's a cool idea
http://www.hauntforum.com/showthread.php?t=19770.
This is from his 2009 haunt. :

He told me to just cut the burlap into strips and then dye the strips with various colors of RIT. Then you slip the strips onto pvc, wood, or rope across your ceiling and it would hang in the path of the visitors. After that you could probably run a large roll of burlap across the length of your hallway/room to completely cover the ceiling. You would have to watch out though because it could catch fire fairly easily. Not sure if that's what you're looking for, but it's a cool idea