I would rule you will need to pay an extra 2000gp for Cold Iron Weapon every time you upgrade its enchantment. This is to say, upgrade a Cold Iron Longsword to Holy Demon Bane Cold Iron Longsword+1 will cost you an extra 2000gp, upgrade a Holy Cold Iron Longsword+1 to Holy Demon Bane Cold Iron Longsword+1 will also cost you an extra 2000gp.
There is an example in F.A.Q., it does not clearly state how it should be done, but I think this ruling is good enough. Cold Iron weapon is almost a must have for PC, I do not see there should be a reason making it more expansive.