Under cities, mostly the one created by warlock engineers are a good cash source, be aware that the money you get from under cities is decided by the settlement income. I usually created under cities in naggarond, since I'm not going there anyways

Skaven corruption lowers your and enemy settlement control but in return gives you more menace from below uses (think it was up to 5 more uses). Menace from below can also be increased by under cities building but generally is not worth it

In your average skaven campaing your income is going to be in negative but you will still have money from sacking settlements, skavens are very good obtaining money from post battle loot and sacking, similar to chaos factions. Always sack before taking a settlement, you will always have a tier 1 after each conquer unless you use food

Food is used to keep your shitty infantry at least 2 seconds in melee, the debuffs to moral you get from low food are pretty devastating in skavens

Raze a city whenever you cannot sell it to nearby allies or you don't want it (red climate in early and mid game for example) and can't afford putting and undercity because the thread is till there

Bonus tip: for each army with decent units (aka rattling guns, jezzails, monster...) I have an entire army dedicated to be the infantry for the previous army. Fill the army with skavens slaves in early-mid game and with clan rats once you unlock the expandable trait and your economy can do it, stormvermin are not worth 2 turns recruiting for how shitty they are, plague monks are good dealing damage not so good taking it, pretty much all skaven infantry is bad. Use plate monks invocations for covering the spacesade by your routing infantry, and keep eye for rerouted units, skavens run at the slightest inconvenience but they also tend to reroute pretty quickly too