The value of weapon enchantments is equal to the total effective bonus, multiplied by itself, then doubled, and multiplied by one thousand gold pieces.
The total effective bonus is equal to all of the enchantment's effective bonuses added together.
A magic weapon must have a +1 enhancement bonus.
Cold iron adds 2000 whenever you get it enchanted.
The multiple striking surfaces of a weapon with multiple striking surfaces, such as the two heads of a two-bladed sword(probably better referred to as a staff-sword as a two-bladed sword could just as easily be a sword with sharpened edges on both the inside and outside edge, which would make for an interesting weapon in its own right...), are treated as different weapons for enchanting purposes and special materials. So you could have them enchanted at different times. It also costs twice as much to have it made master-work, if I recall correctly...
Magic weapons must be of master-work quality.
So, you have a two-bladed sword with a master-work silver head and a master-work cold iron head. You decide you need some magic to over-come damage reduction, so you put the bare minimum magic(+1 enhancement) on the silver head.
+1=+1 equivalent
1x1x2=2000gp
If you did that to the cold iron head it would add 2000, making the cost 4000...
Then you decide that you are not hitting enough, so you increase the enhancement to +3 on the silver head.
3x3x2=18000gp
the enchantment's value was already 2000 so bringing it up to 18000 costs 16000.
You decide to turn the cold-iron head into a demon-slayer. You add chaotic outsider bane and holy(which requires a rare spell) to it, and to make it legal you also need to add +1 enhancement.
+1=+1
Chaotic Outsider Bane=+1
Holy=+2
1+1+2=4
4x4x2=32000gp
cold iron adds 2000=34000gp
Against a Balor it will now bypass the damage reduction, have +3 to attack bonus, and deal an extra 4d6+3 damage per hit...
Wouldn't you know it, now the place is crawling with eladrin and fey. Add Fey bane and unholy to the cold iron head...(two banes on the same weapon is stretching the benefit a little, three would probably not be worth it...)
Bane=1
Unholy=2
previous=4
1+2+4=7
7*7*2=98000
98000-32000=66000
66000+2000=68000
+1 cold-iron (un)holy blade that is a bane to fey and chaotic outsiders, it cost(the equivalent of) 102000* to make but is only worth 98000*...
*plus mundane costs
sometime later you upgrade it to +4 to bypass epic damage reduction on fey and chaotic outsiders, at which point it is the equivalent of a +10 enchantment, which is the maximum, so you can't do any more with it...