Sorry if this has been answered before, but I searched the General Discussion threads and only saw how HAs can benefit from Divine Bond (but not how much benefit can they receive).
So if a HA is a +5 holy weapon, that makes it +7 total enhancement. Can they then be further enchanted with another +3 of enhancements (so long as it doesn't raise the +5 any higher, I know), or is there something I'm not taking into account? Thanks!
If you go back and check the price of a Holy Avenger vs that of a +7 weapon you'll find they aren't the same. That's because the Holy Avenger isn't the same as a +5 Holy weapon. It's got it's own set of limits and powers. To the best of my knowledge there is no way in the RAW to make it more powerful.
However what I as a GM have always allowed is for people to upgrade equipment like that based on it's abilities other than the special ones. So if for example in my game a player wanted to add Keen to the Holy Avenger that is the equivalent of adding a +1 ability. So a +7 weapon costs 96k and a +8 weapon is 128k so to make the HA Keen as well would cost 32k. The Keen HA still costs more than a +8 weapon does. Remember that this is a house rule, so talk with your GM about it.
The special weapons/armor are kinda wierd. In some cases you can reverse engineer the weapon and figure out exactly what it is.
In other cases, like the oathbow, holy avenger, and celestial armor, the item possess a bonus that you simply cannot get anywhere else. There is no similar enchantment listed to use. As such, any attempt to add to these weapons would be a houserule based on how the DM decides to adjudicate these special bonuses.
There are some other wierdness that can arise. For example, if you let a player enchant an oathbow to +5, does the oath special ability let them exceed the +5 enhancement cap(like bane weapons)?
Personally, I would treat the holy avenger as a +5 holy weapon(+7 equivalent) with some secondary special abilities that do not effect the + of the weapon.
I figure the DR and Greater Dispel Magic abilities are akin to adding certain armor modifiers like Energy Resistance. They add to the cost but don't alter the enhancement level of the item. That's the part I'm not sure about, though, but I can't find any rule about it one way or the other.
So in that light I intend to consider the Holy Avenger a +7 weapon that has additional abilities (DR and GDM) that add to its cost (along with it being made of cold iron), which means it could be further enchanted to get it to the +10 max enhancement. I like the idea that not all HA's are equal, even though they are quite rare in my game.