I think that "gold" idea in a post-apocalytic world while viable is not very flavorful and even redundant. In such world people would trade with useful items and information, and I doubt they would be able to use universal monetary system, and definitely not gold. What I am trying to say, why not to replace "gold" with "supplies"* (or find a better name; from now on I will use "supplies" term, but it can be replaced with any flavorful analogue). This would mean that people work tiles in 3 ways: gather food, work to provide "hammers" and salvage to provide "supplies". "Supplies" could be used in trade, to be applied to maintaince (i.e. "maintaince costs from distance to palace" would mean that the longer it takes to reach the city, the more supplies it takes to maintain it and to have it in your nation). Also for me "finish production with supplies" option is much more realistic than "finish production with gold". At least it means that the unit was built using salvaged parts, electronics and stuff rather than created from scratch. How do they do it with gold is a mystery for me. How is the idea?