Price vs. Cost
First, a word on price and cost, and how I'll be using the two terms, as they are very different from each other. You are effectively correct, so this is just me offering more detail and examples.
If you go into a blacksmith's shop, assuming base book prices, the price tag on a Longsword is 12gp for a mundane sword, 312gp for a masterwork sword, and 2312 for a +1 Longsword. A ship's captain might be selling passage from one port to another, and the price he puts on it is 3 days of work on his ship. It's the listed amount that someone asks for something.
Cost is what you actually give up to get the item or service. At the blacksmith's shop, you might have found an old sword of a unique design that the smith might be interested in trying to copy. If the smith decides that that old sword is worth the normal Longsword that he is selling, the cost to you is the old sword. If you want passage on the ship and don't want to work for it, you might pay the ship's captain 20gp to avoid the work. Thus, the cost of the journey is 20gp. In neither case did you pay the price, but there was a cost to you.
In general, price is what is being asked while cost is what you actually pay.
Price/Cost of an Item
A single item can have many prices. The Staff of Fire secreted away in a dungeon has a listed price of "one dungeon crawl". The same Staff of Fire in Fladnag's Wizard Emporium has a listed price of 18,950gp. If you wish to craft a Staff of Fire in your wizard's sanctum, the price is 9,475gp for components, one of your feat slots for Craft Staff, and (assuming you make your Spellcraft rolls on a nat 1) ~10 days of work (assuming 8 hours of work per day).
The cost of the same Staff of Fire is even more varied. For instance, if you have a diplomacy-happy party face, the GM might allow them to negotiate with Fladnag to reduce the cost from 18,950gp to 18,000gp. Similarly, they might allow the party face to negotiate the cost of the components down to 9000 gp, and if your Spellcraft skill is particularly high, they might allow you to cut the time cost by a day or two. As for the dungeon crawl, we all know how varied the cost of those can be. As little as a single Teleport spell, or as much as weeks of camping and slogging through armies of monsters with hundreds or even thousands of gp pumped into supplies.
The main takeaway from this is that crafting items costs more than money. It also costs in-game time, which you may or may not have.
Cost to Craft
Finally, with all of that background information, I can fully explain how much it actually costs your character to craft an item, specifically related to your homunculus.
Your homunculus has a listed price. This is the price if you were to purchase it from someone else instead of crafting it yourself. Since this price can vary, for the purpose of this example I'll say the price is Xgp where X is the listed price of the homunculus without the extra in question.
Homunculi (I believe) count as "magical" with regards to item crafting, and the general rule is that the gold price of components to craft magical items is 1/2 the listed price to purchase. Therefore, the gold price to craft the homunculus is (X/2)gp. Assuming you can easily make the DC to craft the homunculus, the price in in-game days is X/1000. Thus, the listed price of a crafted homunculus is (X/2)gp and (X/1000) days. If you wish to add anything to the homunculus that increases its price to purchase (such as when the text says "Price: +1,500gp"), increase the base price X by that amount. In this case, the price to craft your homunculus with the extra would be ((X+1500)/2)gp and ((X+1500)/1000) days. Assuming you can't negotiate a different cost, this is then the cost to your character.
If you already possess the homunculus, then the cost to purchase the addition follows the same formula. X being the price of the addition, the price to craft the addition onto the homunculus is (X/2)gp and (X/1000) days.
If you were crafting a mundane item, then the rules are a little different. Mundane items require (X/3)gp in materials, but the time it takes to make them isn't nice and simple like magic items. Please consult the Craft skill for details if you're interested, that isn't in the scope of this question.
I apologize for the semi-extensive math for an RPG, but welcome to crafting economics.