When crafting, what combination of "Other Considerations" can I apply to reduce crafting costs?

Question

My question is this: Can I apply a skill requirement (-10%), an alignment requirement (-30%), and a class requirement (-30%) for a total of -70% crafting cost?

The DMG lists options available to players for reducing the gold cost when crafting items:

Other Considerations: Once you have a final cost figure, reduce that number if either of the following conditions applies:

—Item Requires Skill to Use: Some items require a specific skill (such as Perform for a musical instrument) to get them to function. This factor should reduce the cost about 10%.

—Item Requires Specific Class or Alignment to Use: Even more restrictive than requiring a skill, this limitation cuts the cost by 30%.

It is ambiguous because of the wording of the section -- I cannot tell if it is saying:

1. You may either apply a 10% skill restriction bonus or a 30% class/alignment bonus
2. You may apply both a 10% skill bonus and a single 30% bonus from class/alignment restriction
3. You may apply a 10% bonus from skills, a 30% bonus from alignment, and a 30% bonus from class restrictions

Answer 1

The Dungeon Master's Guide on Behind the Curtain: Magic Item Gold Piece Values (282) actually isn't listing options available to players for their PCs to craft custom magic items at a reduced cost. Instead, the Dungeon Master's Guide is providing reasons why a magic item may have a reduced price compared to otherwise similar items and how those prices may have been figured.

That is, when a player pitches a new magic item to the DM, the DM sets the price for that new magic item. If, when the player pitches the new magic item, the player says, "O, and only neutral evil creatures can benefit from the codpiece of intimidation," the DM's supposed to take that into account when determining the custom item's gold piece value.

However, the DM doesn't have to. As a custom item, the DM can set the price at whatever he wants. (For instance, a DM may be hesitant to apply any discount to the codpiece if all the PCs are neutral evil!)

So if you're the DM you're supposed to compare the custom magic item to other similar magic items then, given the custom magic item's restrictions, maybe drop its price, maybe following the guidelines provided in Behind the Curtain: Magic Item Gold Piece Values—or not, depending on the circumstances, the campaign, and the custom magic item. And if you're a player, those guidelines don't matter because the DM sets the gold piece value for custom magic items.

I know that sounds harsh, but creating custom magic items isn't something players are supposed to have a lot of control over because of how custom magic items tend to unbalance campaigns—especially if the chaotic neutral wizard's custom magic item is available at an extremely deep discount because it requires a Concentration skill check (DC 15) to use and can only be used by chaotic neutral wizards.

Answer 2

The stacking would be multiplicative, not additive. So, to work out your example:

Skill requirement (-10%)
Alignment requirement (-30%) (per axis, let's just use one)
Class requirement (-30%)

Skill requirement reduces cost to 90%
Alignment reduces to 70% (per axis, let's just use one)
Class reduces to 70% as well.

0.9 * 0.7 *0.7 = 0.44
1-0.44 = 0.56

Your total reduction is 56%, but is it realistic to play this way?

Let's say you're a high-level Warlock or an Artificer. Crafting is what you DO. At this point, many DMs (myself included) would trust you to have the reins on that item creation - if only to spare themselves the headache of having to determine every single thing.

For one, the key is not to abuse this trust. There are plenty of feats already to reduce your costs. If you are going to add modifiers like the ones you mentioned, make sure they actually become limitations in some way.