This is in the 8.23 version. No idea that what we are going to get in the 8.3
When you do the fort quest, the main benefit it´s not the passive income. It´s the boost in power you get. You suddenly start to be in the lower end of the political actors in Raana. Your power rating can be, without the fort, in high teens/low 20s. The fort can add easily 35-40 to this ranking. You are a faction of 60+ power. About 40% of the power of the Catholic Ahurch or the Aesir Cult. That´s something. Now it will have no impact in the game, but it´s something that will be developed in future updates (I hope).
If you want returning from you investment, invest in business. First you have a bunch of small ones.
Fälth brother's (Watery Eyes), Ikaanos' German caravaneer's guild (Kasey's Park), Stralsund's caravans (Imogen's Barrow), Gero Falkenstein (Kasey's Park), Felton Gang (Stokke Hills). All in all, are just 10.400 of investment and bring you with Rebeca 302$ daily. Plus wife and accountants. You get back your investment in less that 35 days.
Now for the medium sized ones.
Aimee's Goods (Stokke Hills), Belial Guns (Stokke Hills). The cost of Aimee it´s 47.000 (skip the last the last step in crafting, gives you nothing) and the cost of Belial it´s difficult of estimate, because you get the deal bringing weapons, and depend of how many weapons are looted or crafted and how many are you buy. If you get both, you have 280$ extra. (plus accountants)
And now, the two big ones. Walton's Smithery (Redhaven) and Abbot Roo's General Store (Redhaven). 275.000 of investment and 1600$ with Rebbeca, wife and acountants apart. You get your money back in 170 days.
Train your slaves in profitable jobs. 8 slaves, working at the Golden Dragon or an the Academy can bring you 900$ more.
All in all this account for 3100$ (+ accountants and wife) every day. Just for awakening yourself in your nice bed. Other investments (Carroza, Orphanage, May Lin father house, the fort or the churches and Cults) are more for other effects, but can rise this to 5.000$ per day of passive income.
If you want to check there are a save. 4500 of profits every time that the sun rises