My PC (Artifacer, now 6th level) has an existing Iron Defender. It was the bare minimum build for the homunculus, which I believe is 2 HD.
If I want to increase the HD to 3, the Eberron book says that it increases the build by 2,000 GP. Does that also increase the experience point cost? Is it "spend 2,000 GP and the Hit Dice goes up by 1," do I need to spend 80 XP (2000 * .04) as well, or do I have to build a completely new homunculus with 3 Hit Dice?