Well you can look at how the US has done in the middle east for an example of the economic cost of asymmetric warfare. Though I generally agree with your point that it's not going to work out well for the invaded country either unless another state actor comes to their aid. It's also worth noting that the French Resistance was considered important not just for the acts of sabotage and intelligence it gave the allies as they advanced but also in the rapid growth of the FFI after the allies invaded.