I saw a vendor selling stencils for $50, but I assumed they were steel. Steel stencils are the only ones I've ever seen. So possibly they were mylar or something else. Seems expensive for mylar though. Is that what non-steel stencils are made of?
Sometimes they use Kaplon or Mylar cut by laser. Not sure of benefits of one over the other. But the cheap one I saw was a polyethylene synthetic paper, which apparently works also and was cut by regular vinyl cutter.
I've seen Kaplon/Mylar ones for $35 on some US site, and the synthetic paper one was $10.