Gold is also free. I agree that the amount of gold it takes to get a specific affix combo should represent less farming time than trying to get it to drop. But:
If an affix has a 1% chance, then the odds of getting it along with two affixes you want are 1/100 the odds of getting those two other affixes and a third affix you plan to reroll. Each time you use the enchanter, you get just shy of a 2% chance to see that affix. So on average you will find one about every 50 rerolls.
For non-GA items, that means that a decent average reroll cost would be twice the sell value of a legendary of equal item power. If that was the cost, it would be more efficient to reroll a drop with two good affixes, as long as you could find more efficient sources of gold than just selling legendaries (which you definitely can).
For 1 GA items, though, the cost needs to be much much higher. I’m not certain what the ratio is, but let’s say that 1/40 affixes roll as GA. That would mean that if one of the affixes you aren’t rerolling is GA, the cost per enchant needs to be about 20 times higher on average (because either of the two could have gotten the GA). You could discount it a bit, but a 20x discount would just make dropped items with 3 good stats pointless (unless they were 3 GA items).
And, if you balanced it precisely around 1 GA items, you’d be underpricing it by a factor of 80 for 2 GA items. Again, you can give a discount beyond the efficiency of gold farming, but that big a discount really distorts things.
So let’s say that most of the items being rerolled are 1 GA and some are 2 GA and very few if any are 0 GA. And let’s also say that people are more likely to try to maximize the 2 GA item vs the 1 GA ones, so about 1/10 of the total rerolls are happening on 2 GA items (instead of 1/80). Those ones should cost 3200x the sell price of a 925 legendary, on average, while the 1 GA ones should cost 40x. So the overall costs should be 356x, to make it completely equal to the balance on drops.
So somewhere between 15M and 35M depending on the slot, as an asymptotic cost. This would be a huge discount for rerolling 2 GAs but would only be worth rerolling a 1 GA while the cost is in the 200k-500k range, depending on the slot.