1. Calculate DC
2. Pay 1/3 GP in "parts".
3. Roll appropriate Crafting Check.
4. If Check succeeds - the multiply DC x Check x 1/2 x Gold.
In short - if you are crafting a "weapon" and the DC is 15, and the weapon costs 30 gold and you roll a "Craft Weapons" check of 20 - your total is 15x20x.5xgold = 150 gold completed in seven days. Or - 21 gold pieces in one day of work.