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I was playing around with the magic item crafting rules and wanted to make an interesting piece of armor. I haven't designed custom armor before.

The math:

+2 Breastplate: 4,350gp

Continuous spell effects:

Iron Body: 8x15x2,000=240,000gp

Enlarge Person(multiple different abilities):(1x15x2,000)x1.5=45,000gp

Total: 289,350gp

+2 Breastplate of the Iron Golem Aura strong Transmutation; CL 15th Weight 50 lbs 289,350gp

Upon donning this armor your body transforms into living iron and causes instant growth, doubling its height and multiplying its weight by 18, increasing the wearer's size category to the next larger one. If the wearer's size increases to Large he has a space of 10 feet and a natural reach of 10 feet. You gain damage reduction 15/adamantine. You are immune to blindness, critical hits, ability score damage, deafness, disease, drowning, electricity, poison, stunning, and all spells or attacks that affect your physiology or respiration, because you have no physiology or respiration while this spell is in effect. You take only half damage from acid and fire. However, you also become vulnerable to all special attacks that affect iron golems.

You gain a +6 enhancement bonus and +2 size bonus to your Strength score, but you take a –8 penalty to Dexterity as well (to a minimum Dexterity score of 1), a –1 penalty on attack rolls and AC due to its increased size, and your speed is reduced to half normal. You have an arcane spell failure chance of 35% and a –6 armor check penalty, this is cumulative with the armors regular ASF and ACP penalties. You cannot drink (and thus can't use potions) or play wind instruments. Your unarmed attack deals damage equal to a club sized for you (1d8 for Large characters), and you are considered armed when making unarmed attacks. Your increased weight while this armor is donned would cause you to sink in water like a stone. However, you could survive the lack of air at the bottom of the ocean.

CONSTRUCTION REQUIREMENTS
Craft Magic Arms and Armor, Iron Body, Enlarge Person; Cost 144,675 gp

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    \$\begingroup\$ Is there a reason your weight increases by 18 times instead of 80 times? 8 for enlarge person, and 10 for iron body. \$\endgroup\$ – BobTheAverage Apr 18 '17 at 23:39
  • \$\begingroup\$ Since one said multiply its weight by 8 and the other said multiply its weight by 10, I figured your multiplying your weight by a total of 18 times. \$\endgroup\$ – Azazel_ Apr 19 '17 at 0:57
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No, you've missed an important note from the magic item creation guidelines:

If a continuous item has an effect based on a spell with a duration measured in rounds, multiply the cost by 4. If the duration of the spell is 1 minute/level, multiply the cost by 2, and if the duration is 10 minutes/level, multiply the cost by 1.5. If the spell has a 24-hour duration or greater, divide the cost in half.

Both Iron Body and Enlarge Person are 1min/level spells, so the cost of making a continuous/use-activated version of those effects should be doubled. That would give you:

  • 4350gp +2 breastplate
  • 480,000gp continuous Iron Body
  • 90,000gp continuous Enlarge Person (at a CL of 15)

For a total of 574,350gp.

Also, as Bob points out, the weight increase should be 80 times, because the effect is multiplicative - if you had cast Enlarge Person, multiplying by eight, and then Iron Body, multiplying by 10, you should be eighty times heavier. The special cases rules for adding multipliers together where x2 + x2 becomes x3 is only applicable to rolls, as per this question.

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    \$\begingroup\$ "repeated multiplication is additive, not multiplicative"; rpg.stackexchange.com/questions/83188 (3.5 uses the same rules). the final weight would only be 17 times the base. \$\endgroup\$ – tzxAzrael Apr 19 '17 at 4:16
  • \$\begingroup\$ I agree with @tzxAzdael - even if this rule isn't all applicable, answer should explain why. \$\endgroup\$ – Mołot Apr 19 '17 at 6:07
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    \$\begingroup\$ Well, the question @tzxAzrael linked has the accepted answer that explains that special math only applies to cases where you are asked to multiply the result of a roll, which is an abstract concept. In other cases math works the same way it normally does - specifically we are not modifying a roll result, we are multiplying a concrete, non-abstract value that is the character's mass. \$\endgroup\$ – Carcer Apr 19 '17 at 7:25
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    \$\begingroup\$ Is there a reason why you need Enlarge Person to be CL 15? \$\endgroup\$ – Anne Aunyme Apr 19 '17 at 8:10
  • \$\begingroup\$ @AnneAunyme : I think in general items that allow you to use multiple different spell effects have those spell effects at the same caster level. This is explicit for staves, where it's stated in the description for crafting that all spells in a stave are at the same CL (which must be at least 8). The required CL for Iron Body is 15, so we priced Enlarge Person at that CL too. I think it would make more sense (and be much more affordable) were this a limited use item which let you activate the effect once a day, in which case the CL would actually be relevant to the 1min/level duration. \$\endgroup\$ – Carcer Apr 19 '17 at 16:19

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