Compare refilling cartridges versus buying new ones.
A refill kit is a fixed cost spread over the number of refills it holds, so the per-refill cost is what should be compared against a new cartridge. Multiplying both by your annual cartridge use turns that into a yearly saving, and dividing the kit price by the saving per cartridge shows how quickly the kit pays for itself.
Refill economics
Cost per refill = kit price / refills per kit; yearly saving = cartridges per year x (new cartridge price - cost per refill)
It can, and some printers reject refilled cartridges through chip checks. Weigh the saving shown here against the cost of the printer itself before committing.
Because printing volume varies far more than time does. Counting cartridges makes the payback comparable whether you print weekly or in bursts.
Third-party ink can differ in colour accuracy and longevity. For archival or client-facing prints the saving may not be worth it.