The $12B figure[1] is almost exclusively munitions used so far. The US didn't buy munitions and use them, they already had them.
The cost reports (updated as the conflict goes on) will also include payroll, fuel, food, supplies, etc. Everything needed to conduct the war - but much of that is already spent even if not at war.
[1] - https://www.cbsnews.com/news/kevin-hassett-national-economic...
