I've actually pondered this question myself.
I think there are two answers, for me at least.
1)Rifle cartridges
Yes, I can pretty much buy ammo that's just as good as I can load, or darn close... but I was just doing the math yesterday.
Take my .204Ruger for example
$62 for a pound of Benchmark (ridiculous, but that's what it is)
$100 for 1,000 primers (ridiculous, but that's what it is)
$25 for 100 32gr V-Max
Brass comes out to maybe $40 for 500 total rounds, as I can reload them a bunch but I buy Norma, so it's pricey (50 pieces, 10 reloads)
So, 8 cents for the brass, 10 for the primer, 25 for the bullet and call it 25 for the powder (account for some waste).
Total price per round, 68 cents
Retail cost about $1.25 each
My loads are 1/2 the cost of retail.
There's also the availability question... find me some factory .243AI...
2)Handguns
Much sketchier math here.
I can easily buy 9mm, for example, at about $15/50 tax included (usually more like $13.50) Call it 0.30 cents per round
Powder runs maybe 6 cents per round. Bullets, even the cheapest, are 12 or 13 cents, primer is still 10 cents, brass is relatively cheap ($39/500 once fired) but I lose probably 10% every time... so still looking at probably 3-4 cents per load....
So, my loads cost about 30-35 cents per load....
Break even, being generous to reloading costs.
I still reload handgun, just because I already have everything.
All in all, there's no doubt I could have shot for the rest of my life with factory ammo and never touched what I spent on reloading.