Hancock Fabrics close to where I work is having a sale on a bunch of material, some of it 50% off. I was able to get 9-1/2 yards of bloack polyester poplin (perfect blackout material) 60" wide for $1.75/yd. I also picked up 14 yards of pure white polyester (perfect diffusion material) 60" wide, as I can never have enough.
I also got a few yards of poly silk which has a fabulous texture, and five yards of "costume satin" that was cheap-cheap. It's a tiny bit blue, not even noticeable until you put it up against the white.
I think I'm happiest with the pure white poly. That's enough for me to make a few LARGE diffusers in frames (think 10 x 10 feet) with relatively easy sewing.
A hundred bucks. In fabric. That I think is a low-point record for me, and considering what I got, I'm pretty happy.