by far WW2 is the most cheapest hobby I ever did. Took me one year to complete my Federal kit and im halfway done with my Confederate (Need equipment, using Federals) But as for WW2, I completed my kit in 2-3 month. I just bought a last piece last week, a bayonet frog. That's it for equipment. Only one thing missing for me, K98. I've spend around little bit over 600 on this kit. Unlike ACW is already worth over a thousand. XD
But I am sure Vietnam would be cheapest of all. 300 for whole kit.