I've been down this road before. My conclusion was it's not worth the time it takes to farm myself. If I look at how much gil it costs to buy stacks vs farm stacks I have to think about how much gil I could make in the time it takes to farm. Most of the time I can make way more gil in that time than it costs to buy them.
The only exception is if the market is dry... then you really don't have much of a choice.
Mining in Halvung is pretty easy. I used to do it all the time in the 75 days. Use your fastest moving job and you'll learn where all the Mining Points are really quick.
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I've never had a shortage to farm oricalchum ores, i always mine #3 in my mog garden grove, it seems to give more then #1 or #2 (maybe i'm imagining things though). atm its intresting to do VW too for ores with the campaign being active.
I do craft those ingots slowly though, short bursts of one lvl every two weeks.