From the SRD:
Say I have a masterwork cold iron longsword and decide I want to get it enchanted to be a +1 sword. This would cost me 4,000 gp (2,000 for the +1, 2,000 for the cold iron penalty) plus the cost of the sword. Now, if I want to further enchant the sword at a later date do I need to pay the 2,000 gp penalty every time or is it a one time thing to "magic up" the sword?
Iron, Cold: This iron, mined deep underground, known for its effectiveness against fey creatures, is forged at a lower temperature to preserve its delicate properties. Weapons made of cold iron cost twice as much to make as their normal counterparts. Also, any magical enhancements cost an additional 2,000 gp.
Items without metal parts cannot be made from cold iron. An arrow could be made of cold iron, but a quarterstaff could not.
A double weapon that has only half of it made of cold iron increases its cost by 50%.
Say I have a masterwork cold iron longsword and decide I want to get it enchanted to be a +1 sword. This would cost me 4,000 gp (2,000 for the +1, 2,000 for the cold iron penalty) plus the cost of the sword. Now, if I want to further enchant the sword at a later date do I need to pay the 2,000 gp penalty every time or is it a one time thing to "magic up" the sword?