I honestly think that 3% is too much. Owning alleyways is ironically the safest passive income in the game. It requires a very little (but kinda specific) "investment" of a companion that has a neutral or negative mercy trait with 30+ Roguery and at least 5 troops. In total, it requires approx 300 denars (and a bit of game information) for the companion + 100 denars for recruiting 5 recruits at 20 denars each from the town equaling 400 denars total. Our alleyway has no cost, not even the companion and the recruits' daily wages.
The only "risk" of owning an alleyway is the occasional defence event. If we lose the alleyway we don't actually lose the companion (though they might have a small chance of dying, I have no idea), just the troops we put in, which in my example would be 100 denars. The quality of the troops we put in doesn't even matter that much, since the alleyway automatically generates bandit troops (whose type depends on either the area or the culture of the town) and thugs over time for free. Nothing happens if the player character gets knocked out by thugs when trying to take over an alleyway either, we just have to wait a bit to recover our health (although there is a very tiny chance of dying).
This is going to be very sloppy napkin math, so apologies in advance: From my very very basic research, an alleyway had a defence event roughly twice every in game year. If we say that an alleyway will only be controlled 40 in-game days without player interaction (This includes the 5 days on average that we have to respond), and that players will own alleyways in towns with an average prosperity of 4k (this is a low estimate that I'm giving for easy maths' sake), the revenue from alleyways would be 4000*0.02*40= 80*40 = 3200 denars. Assuming that this is the first alleyway we acquired and therefore we had to pay 300 denars for the upfront cost of hiring a companion, our total investment is 400 denars. This makes our profit 2800 denars for the very little risk of losing 5 recruits we hired 2 minutes before capturing said alleyway. If we increased the rate of revenue by 50%, we would have a profit of 4800 - 400 = 4400 denars. It's still not a groundbreaking amount of money by any means, but it has almost no risk. With 80 denars per day, we break even after the alley's 5th day. If you actually think about it, alleyways have beyond ridiculous return on investment. If we want to only make up our investment for the recruits in 1 day, we only need to go to a town with 5k prosperity, which is easily doable in day 1.
Sorry for writing such a long comment and thank you for coming to my TED talk.