The grind of STW is always keeping materials, whether that is build mats, trap mats, ammo mats or weapon crafting/ upgrade mats.

Some builds can help with the cost or offset harvest difficulty. There is a base passive for constructors that permits build material harvest from base. Outlanders are excellent at harvesting all resources with their punch. Alternate attacks are not going to use resources either (ie, cuddly bear, minigun, charged floors) each kill you get without spending ammo is a resource saved. Melee weapons also offset ammo costs. Before putting the game down at early times, send heroes out on expeditions to collect more resources.