I agree with Brian, sounds about right.
When I sold a filter, I stopped at Petsmart (ok, shush) and bought a 3 pack of cartridges for $10. I added that to the price and it was snatched up right away. I felt good too knowing the "novice" fish keeper would know exactly what cartridges to buy in the future and that they had a few to get them by for awhile.
Good luck to you...