If your rinsing your filter media out each week, price shouldn't be a concern.
I just replaced the filter media in my 170 after rinsing it weekly for well over 6 months. I still could be using it, but I felt it was time for a change. but the new is doing just as good a job as my old one.
as long as the blue stuff isn't breaking off and becoming goo, its good to keep using.
30gal; Bala Shark, Pl*co