The Breast Cancer Site store has recycled sari silk yarn on clearance. With shipping, 3 skeins were less than $18. Part of the sale goes to supply mammograms to people who can't afford them.
I recently made a the Unbiased Bag (pattern's on Knitty) using this yarn. I probably posted it here before, but here it is again. It's not the sort of yarn you'd want to use for wearables, but it does make a cool tote bag.