It's been tried before, what yer thinkin' of... Remember CTP2? They had a public works budget that drew from your commerce flow. Points built up, IIRC, on an empire-wide basis and could be applied anywhere. Like a lot of CTP's innovations, it sounded great on paper, but was a total nightmare in practice. Not saying it couldn't be done with the right approach / interface, but right now builder charges seem the least worst option.