I doubt a subsidy is required.
Where you do definitely need a subsidy for is the payment of wages of a bunch of people. This is a scene commonly seen around European council-owned airports, which manage to "employ" dozens of people despite having (in many cases) a very low level of traffic. Union-safeguarded job demarcation ensures that firemen cannot do refuelling, etc. so you need to employ loads of people, and nothing even approaches the cost of employing people (in Europe). And full ATC is expensive - but mandatory for an instrument approach...
But if you were to start a GA airfield from scratch, you would not do that. You would not have any scheduled services and this enables the fire crew to be virtually eliminated. 1 man can do the whole lot including admin (with suitable automation, called a "PC"). 1 other person doing the other stuff.
What is vital is (a) getting the planning permission and (b) getting the rateable values set at realistic levels (a fraction of present-day commercial park rates). The first is theoretically possible (although almost nobody would put enough money where their mouth is to try it) and the second is probably impossible to achieve to a sufficient degree. One would think that a PP restriction to "aviation only" would do it (enabling the rates to be reduced at a tribunal) but I don't think it works well enough.