Can you combine magic items?

[Babbling Bazzar]

Recently I have a barbarian that has a Belt of Giant's strength and a Cord of Stubborn Resolve. I was wondering if I could combine the two belts.

Belt of Stubborn Resolve = 15,000 gp
Belt of Giant's Strength = 4,000 gp
Belt of Physical Might = 10,000 gp

So if I break this down, the +2 to a stat is 4k gp, the combination of the two is +2k gp, and the fatigue effect is about 11k gp.

Would that mean that at combination of all of that be base 21k gp, if possible. Or would a price of 23k gp be reasonable price?

[RumpinRufus]

Each effect, besides the most expensive, is 50% more expensive than usual if you put multiple effects onto a single item.

[Nox Aeterna]

"Adding New Abilities
Sometimes, lack of funds or time make it impossible for a magic item crafter to create the desired item from scratch. Fortunately, it is possible to enhance or build upon an existing magic item. Only time, gold, and the various prerequisites required of the new ability to be added to the magic item restrict the type of additional powers one can place.

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.

If the item is one that occupies a specific place on a character's body, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection +2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5."

[Babbling Bazzar]

Okay, so 21k gp is correct because it would be 9k for the fatigue effect, then 6k for each of the stat increases. Thank you for the information. It was very helpful!