Jokes aside. D2 after ladder reset, might pay off better than if you buy fg with real money (assumed you are from US or have 15$/hour wage). If you play and MF a lot after ladder reset, you can get thousands in a few days, even more if you cash out and sell your char when the market starts to hit ground. (few weeks couple of months after ladder reset, depends on if Blizzard bans the bots, if yes, might be even more time
If you have a lot of fg, you can invest in PoE exalts (?) buy cheap, sell high. It takes time but pays good as I heard.
If you like to have a constant but low fg flow, try Relics of Avabur. You can play it in browsers in AFK mode, but it is dangerous, because they don't like if you sell your stuff for money or FG or anything, so your Relics acc might get banned after the first cashout if you are not careful enough.