Well, the Catch-22 is this.
12 Gauge ammo is cheap to buy and easy to find. It's probably the most expensive to reload, except for the hulls, which you can get for free if you're even slightly resourceful. But 1 1/8 oz. of lead costs more than 7/8 or 3/4, and 20 grains of powder costs more than 15, plain and simple.
28 Gauge ammo is so expensive to buy, and so much cheaper to reload, it's a no-brainer, regardless of component prices.
12 isn't, though.
I reload 12 for trap league. I can load competition-quality loads with top-quality shot for the price of bulk shells at Wally World. But not for significantly cheaper. They're just better.
Unless you have a job making 50 cents an hour, it's just not worth it, when you figure the time it takes. It could be a fun thing to do with your son, but it also might take away from other "quality time".
I'd run the numbers, to be sure. Comparing apples to apples (STS Nitro's vs. high-end reloads), yes reloading is cheaper and you can get EXCELLENT loads tailored to your gun.
However, you have to ask, "Do I care?" My trap loads will bust clays with authority with an IC choke 50-60 yards away. But so what? For having fun on the weekend, do you need perfect patterns? Or can you just change chokes and accept a slightly worse pattern? Will you even notice?
I can't say that I really notice, when shooting. I shoot an after-work trap league, though, and loading my own gives me a bit more confidence when shooting, and it takes away something I could blame if I miss.
For casual fun shooting, who cares?