Hello.
We have some stock in storage that belong to our supplier. We need to have that stock in the system, for planning reason (MRP calculations and so), but we need that the stock has no cost, since it won't belong to us until it is finished, that is the time when our supplier bill it. How can we make a null cost price? we'd need to have the right purchase price for these items.

Suggest putting the item in a different warehouse, and so leave it out of your normal stock valuation. If you mess with the cost/standard cost your costings will go wrong.