We should aim for the mundane cashflow to be resistant to major disruption; either through having a surplus which can be horded and spent cyclically, or by several sources. I think that desire is covered from what MTK has described, so after that any extra coins are just more metal. A Boon of some sort would be good, but I think unless the surplus is consistently large there should be consideration to how to grow the covenant. And significant growth requires investment.
I'd note though a lot of things which I think would make good upgrades for the covenant cost a significant amount of silver, so I couldn't see a scenario where the Magi didn't have a plan for improvement. Getting each project underway with skilled resources is probably more an issue, but again, that is a great problem to have.
- lab improvements, multiple specialist buildings, better living quarters for the folk (+morale), funding of expeditions by companion class characters, a wider breadth of farm stock, etc.
I now this is a more conservative approach, but the last thing I want to worry about is paying the covenfolk, or having to skimp on equipment, or on labs.
Q. How many pounds per year excess do you mean?