But it does, there should be a difference before and after building the guild in city income. I got about 100-200 florins per upgrade. Although it would be helpful if we'd actually see the effects of the guild(s) in the details window.
On a sidenote, does anyone know anything about a limit on guilds? It seems there is one. It's either similar to the family (proportionate to regions), a global limit or limited to ten in general. Not sure if upgrades count or not, but in my current campaign I owned 20 regions at the time, had 11 guilds and I wasn't offered any further guilds at all! Even though I was building tons of guild pts. When I scrapped two guilds I suddenly got an offer again, so either my triggers are yet again fubar or there is some sort of limit. Pretty stupid imho.