Frankly I am amazed that some of the cottage gear makers sell their tarps at so low a price. Someone selling a silnylon tarp for less than $60 is probably losing money on it. As others have shown, silnylon is not cheap. I have found it at walmart for $1.50 a yard, but that is very, very rare. I should probably add another "very" to that. And, I had to sew it myself. On a few occasions, I had to siliconize it myself as well.

You can also tape together a tyvek tarp with homewrap and tyvek tape. You can get the homewrap from building scraps for free, and buy the tape for $10. No sewing required.