Hmmm.... on one hand, not enuff data to give pointed advice. In general however:
- what the heck you need 20K gp per month for? In a medieval village, costs for stuff like road repairs etc. were not that massive. Also, ppl tended to build to last, so upkeep on buildings etc. are also not major cost centers (except maybe city walls, usually not that many other public buildings besides military related stuff like barracks, the central keep, city walls, maybe a bathhouse...), the main costs would be wages for servants, guards etc., well, shift some of the guard duties from hirelings to militia, let the villagers care more for themselves...
- if in a frontier town, stimulate the coming of adventureres... they bring loads of cash, and you can let them loose on the various threats from the wilderniss and the orcs...
- make sure that there is plenty for these adventurers to waste their money on: weapons and armor, supplies, simple magical / alchemical items (potions of healing, low-level scrolls etc.)
- definately reopen the mine.... allow ppl to work in on a concession basis (i.e. they keep half of what they mine, the rest belongs to the municipal coffers...) let THEM take care of any security issues by hiring adventurers etc, only helping out if really necessary (hey, as ruler you have better things to do then act as guards...)
- scout out other forms of resources: other mines, wood, animal furs etc. how is the food situation, can sell surplus or do you need to import...
- maximize profits (also in terms of taxes) by reselling finished products in stead of raw materials, up those margins -> attract skilled labourers to settle in your city to produce quality materials -> high value / low volume, easier to trade...
- repair the sky-ship, is a pretty safe (well, except when there are unfriendly dragons about) way to establish a trade route to the nearest (major?) city, especially if mainly smaller, more valuable things need to be transported...
- introduce licenses, for example on services such as spellcasting, a special adventuring license, the right to carry arms...
- if the town provides protection, then all who enter must pay an entry fee (for the upkeep of the walls)...