Vahktang said:And you are supposed to multiply the result of the DC for how much value is produced each week towards the total value.
Explain how if you make it by 12 (the first example) takes a longer time than making by 2 (the second example)?
First, your DC is not a number you just select. it represents how hard it is to make, the quality of the item. it is not a separate thing from the items price. A DC 28 to craft dagger will be a much nicer thing than a DC 18 dagger. It will cost more, and thus not be a quicker make. The error you are fabricating is assuming you can raise the DC without also raising the price.
Second, if your Gm wants to allow you to go against higher DCs, to possibly represent working harder or using more sophisticated techniques but still only trying to make a normal item, thats also logical and makes sense. The guy using the easy, old, reliable methods which require less skill takes longer than the better craftsman who uses more refined techniques and produces the dagger in shorter time. better craftsman, quicker result.
I just do not see any logical error here.