When you add special abilities to armor (specifically the ones that have a "+X" price, not ones with a +X bonus), do the item creation rules for adding additional magical abilities apply?
Or can you keep adding as many additional special abilities as you want at the listed price?
It costs what a weapon or armor with that bonus costs according to the chart. +1 costs a certain amount, +2 costs a certain amount, etc....
If you have a +1 breastplate, then add an ability that 'costs' +1, then GP wise, you have a +2 armor. Add another +1 to the enchantment, or a +1 ability, and you have a +3 armor, with corresponding cost.
I do believe he's talking about the extra cost for things like energy resistance or shadow.
As far as I know these do not add on a percentile to the cost of the item in the same way as other magic items do.
Glamered, Slick, Shadow, and those types of special abilities do not affect the overall + of the armor. You simply pay the listed amount and add the special ability.
To clarify...
The question is not about special armor qualities that count as a "+" to the weapon's cost (i.e., a +1 weapon counts as +3 with the bane quality).
The question is whether adding special qualities that purely cost additional GP (e.g., +3000 GP) have any limits and/or whether adding multiples increases the costs (like the normal rules for adding magic abilities to an existing magic item).
There are no limits, other than gold and time. It does nothing to the base cost that is based off + bonuses, such as +1 1000gp, +2 4000gp, +3 9000gp, etc.
If you want Energy Resistance (Acid), pay 18,000gp. If you later want to make that Improved, pay an additional 24,000gp to get to the total 42,000gp price. Later you can make it Greater by paing an additional 24,000gp.
All that can be added to a +1 suit of armor and when you want to make the armor +2 or add Light Fortification it only costs 3000gp more to make it a total of +2 bonuses as the chart shows, even though it may be Greater Energy Resistance (Acid), Glammered, and Greater Shadow. The last few just make it a really expensive suit of armor but do not affect the price due to + enhancements or special abilities.