I've seen those in the dollar store and thought the same thing, I'd try them if I were on a budget. I used those safety blankets in my first cabinet build, not bad but difficult to get a clean/even installation and they rip too easy. Have since switched to an insulation called DuroFoam that HD carries, about $10-$12 per 4'x8'x3/4" sheet and easy to work with, crazy reflectivity on the one side, it's like a mirror, easy to work with and cheap. Re-lined my veg cabinet with it and built my flower room walls with it as well.
Before & After pics of my veg cabinet, first with the blanket liner, then after the re-line with DuroFoam. 3rd pic is my flower room just after I finished the walls and fired it up for the virgin run: