#1, assume that anything you craft these days is for npc resell only. Yes, some items you can AH slowly and items that do sell regularly are generally crafted by 100+ skill players as HQ synths.
#2, research the crafts. Yes, most of the guides are old now, but they still have a basic path to follow. Look through all possible crafts in a 10 level range and see what you can get, costs, etc.
#3, past 50 skills slow down. Before you may get a level per 1-2 stacks of materials, now it generally is 2-5 stacks a level. Use that as a gil guide. If you need 5 levels of skill, it takes 3 stacks of materials per level, and materials cost 100k per stack....already at 1.5M just for those 5 levels. You may get all that back, some back, none back...just depends.
#4, if you really want a 100 craft, do it. Farm dynamis, salvage, VW, Delve, whatever you can do to make gil and put that into the craft. Too many events these days where you can't at least do 1M in 1 hour. Craft, farm, craft, farm. Ten levels of crafting a day is easy to do unless stupid lack of materials comes into play. I did 70-91 alchemy in 2 days this week; now I'm gathering materials and getting distracted by EMPY stuff.