It is very difficult to say what are you doing wrong without seeing your model... but... for example...
If costing is calculating the cost of material using the mass of the part and you link the costing to the net mass (weight of finished part) the cost of material will be less as you remove more material from the blank. Them if labor cost is less that material cost, part cost will go up. Cost of material should use the mass of the blank (before processing).
There are other possible problems...
The model I'm using is one that was downloaded from this forum since the tutorial was missing the part file.
Where would I look to change the order of things to prevent not removing material from increasing the cost of the part?
I'm guessing the Costing thing is troublesome, as I see lots more questions than answers.