You're paying more because of how good of a job it does. If you want a cheaper alternative, I think most people reserve poly filter for when they're trying to remove something from the tank (copper, ammonia, meds etc), the rest of the time it's polyfil.
It's the same thing people use for filling stuffed animals. I don't know what brand I have right now, but I buy it in rolls and just cut it to size. Even with filter baskets that required changing 3ish times a week, it would still last *months* and only costs like $15.
Just as an example, and I'm not saying you should buy this one (again, I don't know what brand I have at home), but something like
this. Your LFS may have it on hand. Both of mine carry something similar to that.