Can i enchant, lets say Blade of Binding to +2? How to calculate the cost?
I would say no, not by RAW. Because it has a unique ability.
If you asked in my game, then sure:
original price: 12350 - falls between +2 and +3, so I would make the cost increase the same as from +3 to +4; 32000 - 18000 = 14000
new price: 26350.
Thanks (:
When trying to further enchant such items, you should try and break the price down into plus-equivalent and flat-rate prices, with the plus-equivalent being the more important of the two. So a Blade of Binding breaks down to either:
- MW greatsword with a total of +1 (2,000 gp) plus another 10,000 gp
- MW greatsword with a total of +2 (8,000 gp) plus another 4,000 gp
The new price would then either be 18,350 gp or 22,350 gp. I'm not sure which I'd got with, but I think most people would go with the latter. (If the price falls between +2 and +3, it's probably a +2 item with some extra, so you should probably add the difference to get it to a +3 item.)
Edit: Alternately, you could consider it a +2.5 item, which works out to 12,500 gp plus the MW greatsword. Not exact, but workable. Then the upgraded item would be 24,850 gp. (Not too surprisingly, about half way between the +2 to +3 price and the +3 to +4 price.)
Specific/named items are unable to be enchanted further in PFS.
In a home game, you are getting into custom magic items, which requires the GM's approval. The guidelines in the book aren't hard and fast rules, they are there for the GM if they decide to deal with non standard/published magic items. They give a rough estimate but have always stated that items should compared to other similar items, because everything created through those guidelines isn't 'equal' or 'balanced.'
So to answer your question, the answer is 'ask your GM'. Anything else is someone's houserule.