How do I calculate the difference when I'm upgrading an item?

[5w337x7007h]

I'm thinking about getting a Belt of Dex +4, and I want to eventually add a +2 to Con ontop of it's current effect. How would I figure out the new cost? The +4 belt is 16000. if I multiply the lower item cost by 1.5 (4000 x 1.5 = 6000) Would the cost be 22000?

Assuming the majority of the price would be for the +4 (16000gp) and, if I remember creation rules, the +2 enhancement cost would be multiplied by 1.5 before adding it onto the belt. Which the original price for a +2 belt is 4000 multiplied by 1.5 is 6000 + 16000 = 22000.

Am I right?

Using an item already made;
Belt of Physical Perfection +2.
+2 to Str, Dex, and Con.
Each individually is 4000.
4000 x 1.5 = 6000, so 4000(Str) + 6000(Dex) + 6000(Con) = 16000.

[skypse]

As far as I know, you cannot modify an item in such a way. You can always make a Belt of Incredible Dexterity from +2 to +4 paying the normal cost, but I don't think you can make a Belt from +4 Dex to +4 Dex and +2 Con. I might be wrong but I remember my DM IRL explaining to me why I can't do it and not letting me do it generally.

[Raezeman]

i think you might be thinking about some 3.5 rules from the magic compendium. There, if i'm not mistaken, you could add certain effects to magic items for a increased price. I'm not sure if there is something like that in pathfinder (less experience with pathfinder than with 3.5) but then again there are the basic items that give bonusses in 2 attributes, belts for physical attributes, headbands for mental attributes. Ofcourse, whether or not there rules exist, you might want to check with your dm for this, as not all allow this stuff.

[Firest Kathon]

OP is correct, the item would cost 22.000 gp. See this table (http://www.d20pfsrd.com/magic-items#TOC-Magic-Item-Gold-Piece-Values) for Pathfinder magic item pricing.

[Ninjaxenomorph]

While it does technically work, you should just be able to change your belt to +4/+4 without having to worry about too much pricing issues. Several FAQs support this.