Hello!
If the armor is already +1, adding the champion enchantment to it would make it an effective +2 armor. You would need to pay the difference, which is 3000gp.
Hi,
I want to enchance my Fullplate +1 with the Champoin Enchancment.
Does it cost 1000g extra since it states +1?
As CampinCarl9127 says, if an armor already is a +1, adding a +1 property does not mean you pay the cost of a +1 armor. What you do, is you add the numerical enhancement bonus of the armor (+1) with the special ability enhancement bonus you want to apply (+1), and it becomes an armor with that high of an effective bonus.
The armor table here will tell you the price of the respective enhancement modified bonus your armor becomes. Since Champion is a +1 property, and you already have a +1 bonus, it becomes an effective +2 armor.
Now, that doesn't mean you have to spend 4,000 gold to make it an effective +2, that's not what I'm saying. Here's an in-book example involving weapons (which function much the same as armor, but cost twice as much to enhance):
The cost to add additional abilities to an item is the same as if the item was not magical, less the value of the original item. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 longsword.
So, let's take that bolded part and extrapolate it with your armor.
The cost to add additional abilities to an item is the same as if the item was not magical, less the value of the original item. Thus, a +1 full plate can be made into a +1 champion full plate, with the cost to create it being equal to that of a +1 champion full plate minus the cost of a +1 full plate.
This tells us that for you to transform your +1 Full Plate into a +1 Champion Full Plate, you take the price of a +1 Champion Full Plate (5,650) minus [-] the price of a +1 Full Plate (2,650). Simple math says 5,650 - 2,650 = 3,000 gold for you to make this adjustment.
**EDIT** Miscalculated Masterwork costs. Fixed that.