I've read the rules concerning custom magic item creation, but I have a few questions.
1) Does it cost extra to make an item using a different slot; such as an amulet of speed instead of boots?
2) If I have a belt of incredible dex+2 (4000gp) and want to combine it with the effects of a belt of tumbling (800gp), do I multiply 800 by 1.5 and add to 4000? Do both get multiplied by 1.5?
3) If I later want to increase said belt to a tumbling belt of incredible dex+4, would it only cost an additional 12000gp? Or do I have to multiply this by 1.5 too?
1) Yes. The Pathfinder rules on this are at best vague, but generally, yes. Simple example is Helm of Teleportation vs. Boots of Teleportation. Exact same effect, Helm is +50%.
If the slots are both related to the ability (and perhaps also of roughly equal "slot value") then it doesn't need to go up - GM call, as all of this is in the end.
2) The most costly ability is not multiplied by 1.5. All other abilities are.
3) It would cost an additional 12000gp.
Mostly agree with Majuba, with one exception:
2) When improving an existing item, the second and all subsequent abilities are multiplied by 1.5. When crafting a new item, starting with the most costly is the most cost-efficient. (Note that I agree with the answer to 3.)
I don't see that as a good policy (same item, different costs to create), but you are correct, that is the RAW.