You should rarely have to "restuff" a cartridge. Most users seem to be able to refill a cartridge core about six times before it begins to look dirty. The core can then be removed, soaked in water, and squeezed dry and clean on a paper towel. Put it back in place and refill with your favorite
e-liquid. It's good to go another half-dozen refills.
Treated this way, replacing completely worn out and used up cartridges is not a significant expense. At some point,
buy new ones and begin your refill-reuse cycle all over again.
If you really want to save a few cents, pull out the used up cores and place them in a hypodermic syringe. Squeeze the liquid into a vial. You'll be pleasantly surprised how much liquid you get from a dozen or so "used up" carts. That liquid can then be used to refill more carts.
This works best with larger carts, like those for 801 penstyle e-cigs, which I personally favor. The smaller the e-cig, the harder everything is!