Harbor Freight sells a cheap-o fire blanket that I recon would atleast last a few trips:
6 Ft x 8 Ft Fiberglass Welding Blanket
I buy a few 'nicer' blankets for torching/demo jobs a year from a local welding supply shop, but they're on account and I've no idea what they actually cost. IIRC it's around $90 though.
Something I do when torching or cutting directly over a blanket is to soak it with water. Cuts down on the little pin holes and such, and generally makes em' last a bit longer.