If my income is under a grand per yea, I invest in the economy. Is not a hard rule... but it means that I have a little buffer in case things go wrong. I also try to reach thresholds if 1000 and 10,000 'in the bank' and then keep them.

My most succesful game to date is with a not-very-expanded Danes - just took the Baltic rim - raking in 10,000 per year with a treasury of 300,000. Rock on.

But under these circumstances, I am still over domestic budget to the tune of about 5k per year, if my trade broke down suddenly. With a big enough war chest though I can likely ride it out.