Shamelessly stolen from MTGS flavor forums because they can't have a quality discussion there.
For the guilds to exist and function, they require members and in most cases workers. While these workers may be devoted to the cause, they do require at the very least food, clothing and shelter. It is hard to work and fight for a cause if you are starving. Feeding, clothing and equpping thousands of guild members is not cheap. Let's take a look if the guilds are in any way plausible.
Orzhov is easy. The entire guild exists for the purpose of gathering wealth. Also, it helps that a large portion of the workers are either dead and/or working off some humongous debt.
Golgari seem at the very least plausible. They perform services for the community, which must render some income. Especially when the raw materials for their wares are either free or possibly even garner an income themselves. Half the work force consisting of insects, fungi and zombies tends to help the cost structure.
Dimir can charge a lot for their services. Assassination and theft of otherwise unattainable items or information does not come cheap, even on Ravnica. Blackmail using said information also tends to be lucrative. Still, a bit of stretch as they do operate in a niche market.
Gruul clans don't really have a need for money. As long as each clan hunts or otherwise feeds itself, and occasionally does a raid for other supplies everyone is content.
Rakdos are similar to Gruul in that there isn't really a centralized structure of employment, nor are the members concerned with long term savings. There will always be some business venture of dubious legality to gain some money, or someone will rob a butcher or market stall for rations.
Selesnya is also a cult, which is helpful from a budgetary perspective as the members are not really interested in personal gain. Selesnyans may be growing all the food they need. In fact they may be growing shelter too.
Izzet does provide services for the city and for the other guilds. It seems that this income is not big enough to fund both the maintenance of said services and whatever schemes and research Niv-Mizzet has the scientists working on. I'd say plausible, but only barely.
Simic seems fishy, if you excuse the pun. There isn't really a market for shamblesharks and frog crocodiles. Even if there might be some income from tourism and baths, this hardly seems enough to pay for useless experiments.
Azorius is an unlikely proposition. Running a huge bureaucracy with all sorts of scribes, clerks and magistrates is not cheap. There might be a variety of fines, fees and levies, but even with a large scoop of bribes it seems to me like Azorius is impossible. Unless there is some city-wide system of taxation for the funding of the guild, of course.
Boros seems dubious. Having a large standing army is awfully expensive. Not only does it have to be fed, but weaponry and armor is really expensive. Boros has no discernible way of earning money. Of course, Orzhov might want to temporarily hire a battalion to keep Gruul at bay, but it is kind of hard to be champions of justice if someone else is paying all the bills.
Lets hear it fellow armchair economists. Discuss away.