As this thread is in the shotguns forum, I am assuming you are referring to shotgun shells, not cartridges.
It has been said many times on here that reloading your own shells offers minimal returns because of the high price of shot as of late. I have never loaded my own shells, nor do I have the equipment, but I have seen the comments on here, and a lot of shotgun reloading gear for sale recently.
Cartridges on the other hand are an entirely different story. Factory ammo costs about twice what you can reload for. You can load premium bullets for the cost of basic factory ammo, and if you really scrounge (cast your own bullets, have a cheap source of lead, etc.) you can often do much better than that. As an example, factory .30-06 costs about $20-22 for a cheap box these days. That's over $1/round. Basic .30 Cal. Hornady or Speer soft point or FMJ bullets (equivalent to the bullets used in cheap .30-06 federal bluebox or remington UMC ammo) are ~$30/100, so $0.30/each. Add another $0.20 cents/round for powder, and another $0.03-0.05 cents/round per primer, and you're looking at around $0.55 cents/round.
I haven't included brass in the calculation, because I am currently reloading my factory ammo empties, but if you have to buy brass, you're looking at about $30/100, so $0.30/each. However, brass can be used more than once, and a lot more than once in some cases (especially straight-walled cartridges like pistol cartridges). If you use it twice, you're looking at $0.15/shot. 3x, it drops to $0.10/shot, and so on.
Now you need to decide if it is worth your time. Some people try to factor in their time as $x.xx/hour to this cost, but since I don't reload while I'm at work, or should be working, for me it's not realistic to add this in as a factor. Reloading in itself is an enjoyable way to spend a snowy/rainy evening for me. I don't get paid to watch TV or read a book either

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