291 rings (probably the most popular slot this tier with tier slots making a return), are currently at 39k on Frostmourne.
To make a 291 ring, you need to craft a 235 ring + a vestige of the eternal.
There’s different mats to make one, but I’ll focus on the JC mats since we’re looking at a JC legendary.
The Vestige requires:
2x Essence of Rebirth (760g)
2x Essence of Torment (670g)
2x Essence of Servitude (180g)
2x Essence of Valor (76g)
2x Progenitor Essentia (6200g)
So the Vestige alone sets you back about 7.9k (and some change).
Now IIRC, you have to craft that legendary at rank 4 to upgrade it to 291 so that requires:
80x Shadowghast Ingots (3840g)
40x Essence of Rebirth (15200g)
40x Essence of Torment (13400g)
40x Orboreal Shards (4000g)
And you’re looking at 40.3k to craft a legendary from scratch.
So if you bought all the mats off the AH, you’d actually LOSE gold trying to craft one of these rings. The only real way to turn a profit on them is presumably to mash the ore yourself to get the Essences since that’s the big money sink.
So lets say you buy ore. You need 42x Essence of Rebirth AND Torment. The math works out to about “100 Elethium Ore = 7 Essences of Torment + Rebirth + other largely unprofitable crap”. So you’d need roughly about 600 ore per ring.
Which at 48g each, is 28.8k gold. Or about 1500g worth of savings, plus whatever gold you get from selling the gems you also got out of that ore.
Which gives you about ~100g of raw profit per ring craft, assuming you don’t get statistically unlucky on prospecting.
Oh, and this is crafting rank 4’s in a vacuum and ignoring the absolutely stupid “rank up” system that legendaries use where you craft legendaries that stand virtually zero real chance of being sold to unlock the ranks that actually sell.