(With only 6 sub farms of my own, I fall into the make fuel come direct from credits camp.) However, importantly.
Looking out into one of my wards right now....
You want subs? Are you in NA or EU?
1. Buy a second account for one month to form an FC on Materia. Make 4 characters on each of the two accounts. Leave all at level 1, except your sub alt.
2. Form your FC and start levelling to 6 (For me, the fastest is to buy a warrior class boost and expansion skip on the sub alt then run Shisui ~80 times solo)
3. You can dop your second account sub as soon as the fc is formed.
3. Pay for the 8 extra retainers (you absolutely need these to fund your ceruleum tanks)
4. Level your sub alt character and FC leader to 2nd Lieutenant. Yay squadrons.
5. Level your miner to cap
6. Level all 10 retainers to miner cap
7. As soon as you get FC 6, buy one of the few hundred empty plots, small is best as you can use your free 1 million gil for level 30 in Materia. Goblet is best as it's the easiest to get a level 1 mule into.
8. Start your sub grind. You need cobalt, mythrite, and darksteel mainly. Crafting crystals. A lot. Your subs need to be on this as soon as they can. In the first few weeks keep the 10 retainers very busy and craft, craft, craft
9. It will be about 6 months wait now. Keep your 10 retainers on 18h ventures, trade in the blues. Buy venture coins. This can be a bit of a pain until you get it rolling.
10. Log in every day without fail, do your 10 retainers, go into the dungeon, send out the 4 subs (do your calculations on what route will give the move XP)
11. Have one retainer on dark shards.
After those 6 months, you should have 100,000 a day coming in for those 10 retainers you are paying for. Gather it up, every 6 months to a year, pay a month on your second account and do the 1,000,000 gil trade shuffle to exfiltrate the gil back to your home DC.
Congrats you are now earning 30 million gil a year solo for no work (apart from the daily retainer grind) That 30 million costs you an extra $192 a year. So.... 200,000g / US$