Where does all the excess Resource go?

[Agema]

If resources are anything, it has to be raw materials. You can hardly argue that they can represent labour and time when a grassland (population 15k) has significantly less resource than a mountainous land (population 1.5k).

I wouldn't mind a pretty simple 1 gold per 10 (20? 30?) unused resources in terms of, say, the resources are actually being turned into trade products and general enrichment when they aren't building troops. It could make the Productivity scale interesting - remove the income bonus or penalty, and your bonus can turn into money anyway.

However, both paragraphs assume that things have to make 'sense' in a real world situation. Resources are an abstracted concept designed to fit the game's strategy, and when you muck with things integral to the strategy, that's an awful lot of rebalancing to do.