I use something similar to the polyfill pad you have. I just buy it in a big bag (pillow stuffing). I've been using it for years in my filters. I still have the same bag from probably 5+ years ago.
I like the loose stuff because you can shove it in any type of filter without trimming. Also, I use it around my intakes when I have fry (prefilter). Just wrap some around the intake and put a rubberband on it.
I agree with you though, why pay ridiculous prices for the manufacturer's media?