I've never seen this mentioned in any post, so I'm posting it: not only you can queue up expensive buildings to keep under the 50k threshold, but you can, at the same time, queue up expensive units that take 2 turns to build (1 turn-buildable units are no good if you don't want to actually build them) mainly wardogs, as they are easy to get and they do not require a lot of recruits (I must warn you that queing up units takes out not only denari, but also people). 10 wardogs = minus 6800 denari, which is something, if you are desperate to keep under 50k. Just pay attention and check your unit building queue every turn if you don't want your cities to become Dogville.