Seems like the cost in ammo is related to the amount of items placed, which makes paving surfaces much costlier than expected. In my opinion it would make more sense if ammo consumption was time based instead.

placing tiles is 1/4 the cost of placing anything else, there's just so many tiles that it adds up really quickly. I may end up changing that to 1/8, the tile placement buffs are relatively new and haven't really changed since implemented, but I also figure that since placing tiles by hand is always going to be so much faster, the only real use the shotgun has for placing tiles is fine tuning edges or intricate designs.