Here's Path Of Exile Map Farming Guide
In Path of Exile, is it better to farm t11-12 for profit or invest more into t 13-14-15 for map selling profit? Ideally you are not buying them, but you'll need a pool of 10-20 to start probably. Don't forget poe trade currency is important in game.
4/5 sextant + chisel + 3 atziri frags + zana bloodlines + alch/scour to 80% quant is what I do for my T16 guardian maps, and I am able to sustain them with 100% atlas completion.
You can vaal them too if you want, I don't find it to be worth it with the cost of vaals, and how random they can be with re-rolling my good maps. You can also do elder/shaper influence if you're done your Zana quest, but I'm actively avoiding those influences so I can see if I can sustain without them (I can).
It does work out when you consider the cost of the maps that will be dropping for you. T16s are worth 25c, T15 15c, and T14 8c, and the others below can still be worth something.
You don't want to be buying these maps, 100c can either get you 4 T16 maps, or you can get 16 red sextants, but buying the maps doesn't give you all the extra drops that the sextants do, so unless you are forced to, you want to avoid paying actual currency for the maps. Hence the investment.
Here's what 3 maps will cost me in a worst case scenario: 10 red sextant - 60c (for rerolls, because yes we are rerolling useless stuff) 12 chisel - 6c 9 atziri fragments - 18c Alch/scouring - 15c Zana Bloodlines - 9c total - 108c.
If I get even one single T16 from each of these maps, I've already made back 75% of my investment (this does happen, one average, with this setup). Then you add in all the other red maps that drop (lots), the raw currency, the chance at a nice ilvl base, the guardian fragment, and the other chaff that drops, and you're looking at a nice tidy profit while being able to maintain T16.
You do need a decent currency base (500c + 20/30 red sextant + 100 chisel) to begin, but you can honest recoup so much you'd be surprised.