Tried to update manufacturing costs but was auto-denied

Hi there. I handmake my products to sell through Amazon and try to estimate my earnings based on what I would pay myself per hour (to be at least minimum wage) plus material and packaging costs. In March, Amazon are only reimbursing manufacturing costs for a damaged item. However, Amazon estimated these costs to be a lot lower than I calculate it to be and denied my request to change this value. How could I gain proof about my own manufacturing costs and have this changed successfully? Has anyone successfully changed this? Thanks for any help.