You are convoluting the real cost. The minimum cost of frags is nothing but FREE stam! So the cost of making single skill pots for 30mins, in gold, is actually only 6k. 4k less than the guesstimate that had been given. If you want to buy extra frags, sure go ahead and use the numbers you threw out. That is EXTRA.
Ok, assuming you never have to buy fragments and therefore your only out of pocket cost for that particular potion is the 6k gold, what about all the gold costs to level up your Composing, not to mention your time? There is no telling how many millions of gold I have spent just leveling up my Composing (I estimate at least around 40 million) and I'm still only level 55.
If your time is meaningless to you and you care nothing about recouping your investment costs to level in Composing then by all means, sell your potions for the gold cost. I would be more than happy to buy potions from you at that rate.
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