Stat budget refers to how the values of stats are set for an equippable item of a given item level.
Terms and definitions
- StatValue — the amount of a given stat on an item
- StatMod — the weighting given to the stats
- ItemValue — the total modified value of the stats on an item
- SlotMod — weighting based on equipment slot
- ItemSlotValue — ItemValue modified for the item slot
- ilvl — the effective level of an item
StatMod
All combat ratings have a StatMod of 1, as do Strength, Agility, Intellect or Spirit. Stamina has a StatMod of 2/3.[1] This appears to still be the case in Cataclysm: a primary-color level-83 gem will give +60 Stamina or +40 to another stat, and a raid flask will give +450 Stamina or +300 to Strength, Agility, Intellect and Spirit. Comparing 
[Dragonscale Leg Armor] to 
[Powerful Ghostly Spellthread] also indicates that the StatMod for attack power is still 1/2.
Additional armor had a StatMod of 1/14 as of Patch 3.2.[1]
The post referenced above doesn't give a StatMod for spell penetration, but the stats on 
[Stormy Ocean Sapphire] and 
[Vivid Dream Emerald] suggest its StatMod is 4/5.
Historical Overview
During the original reverse-engineering of the individual "cost" of each stat, Hyzenthelei set out assuming that +1 to Heal had a cost of 100. He then compared items of similar item level and comparable stats and thus arrived at the first version of the StatMod table (which stood the test of time very well). Later, during a presentation by Blizzard at BlizzCon, a slide revealed that 1% Melee Crit = 14 points. This confirmed Hyzentheleis table (1% melee crit was indeed 14 times as expensive as one point of a basic stat), but at the same time gave a benchmark for the proper scale — rate basic stats with one point.
With WoW 2.0, the table was reworked, some properties (most notable Stamina) changed their price, and others were replaced. In particular many combat-related stats were changed to combat ratings. All existing items were updated to reflect this change. This simplified the table, because Combat Rating points are worth exactly one "item point", the same as base stats (exc. Stamina).
Weapon DPS
Weapon DPS is inherent to the iLevel of the item: thus as more stats are assigned, the higher the DPS will be.
Weapon DPS follows the following formulae for Burning Crusade items:
| Weapon Type | Uncommon | Superior | Epic |
|---|---|---|---|
| One Handed Weapon | iLevel<97: = iLevel*0.3448 + 16.7552 | iLevel<97: = iLevel*0.4350 + 15.8250 | iLevel<138: = iLevel*0.45 + 36.1 |
| iLevel>97: = iLevel*0.6333 - 10.7 | iLevel>97: = iLevel*0.7488 - 14.4905 | iLevel>138: = iLevel*0.6 + 15.5 | |
| 2 Handed Weapon | 1 Handed * 1.3 | ||
| Ranged (Gun/Bow/xBow) | iLevel*0.5 + 1.4 | iLevel*0.58 - 0.3 | iLevel*0.4047 + 32.84 |
| Thrown | iLevel*0.5542-8.8045 | iLevel*0.6191-6.9569 | iLevel*0.4047 + 32.84 |
| Wand | 1-Handed Melee * 1.77 | 1-Handed Melee * 1.80 | 1-Handed Melee * 1.83 |
Weapon spell power
Spell power on a weapon is not part of the item's stat budget.
In the case of a melee weapon, the weapon gains "free" spell power in exchange for having less DPS than other weapons of the same slot, quality and item level. For example, a level 359 two-handed epic weapon has either 623.3 DPS, or 213.2 DPS and 1955 spell power. This allows a weapon upgrade to provide the same DPS increase to a caster as to a non-caster, while ensuring that a caster weapon is rarely an upgrade for a non-caster.
At one point, spell power was gained by "sacrificing" DPS at a fixed ratio. However, in Cataclysm this is not the case: one-handed caster melee weapons have the same DPS and the same spell power as two-handed caster weapons, which means they are giving up less DPS compared to their non-caster counterparts.
Wands didn't follow this rule before Patch 5.0.4, when the ranged slot was deleted. Until then, wands had no spell power. They had more DPS than other ranged weapons of the same item level, but gained no extra damage from attack power or spell power.
Armor Values
Armor value is inherent to the item and dependent upon its iLevel. Additional armor can be purchased with normal stat points: gear with additional armor will show the armor value in Green text. However, for most pieces the armor value is given without the use of any normal Item value.
The maximum armor for a particular iLevel piece is calculated using the formula below, and then multipied by the Armor SlotMod to calculate the armor for that particular gear slot. Blue/Superior armors have 1.1 times the armor value of a Green Item, and Epic items have 1.25 times the armor of a Blue item (1.375 times a green). Shields have a separate formula of their own and have different multipliers between gear qualities as shown below.
| Armor Class | Uncommon | Superior | Epic |
|---|---|---|---|
| Cloth | iLevel*1.19 + 5.1 | Green * 1.1 | Blue * 1.25 (Green * 1.375) |
| Leather | iLevel*2.22 + 10 | Green * 1.1 | Blue * 1.25 |
| iLevel*4.9 + 29 | Green * 1.1 | Blue * 1.25 | |
| Plate | iLevel*9 + 23 | Green * 1.1 | Blue * 1.25 |
| Shield | iLevel*85/3 + 133 | Green * 1.22 | Blue * 1.28 |
Armor SlotMods
Each particular slot of armor receives a slightly different percentage of the maximum armor calculated above, according to this table:
| Armor Slot | Multiplier | Fraction |
|---|---|---|
| Chest | 1.00 | 16/16 |
| Legs | 1.00 | 16/16 |
| Head | 1.00 | 16/16 |
| Shoulders | 0.75 | 12/16 |
| Feet | 0.75 | 12/16 |
| Hands | 0.75 | 12/16 |
| Waist | 0.75 | 12/16 |
| Wrist | 0.5625 | 9/16 |
| Back (Cloth) | 0.5625 | 9/16 |
Capes are always cloth armor.
Using these two formulas: an iLevel 115 Blue quality Mail Bracer would have armor of:
(115 * 4.9 + 29)Green * 1.1Blue * 7/16Wrist = 285.14
Compared to the blue bracers, all have an armor of 285: http://www.wowhead.com/?items=4.3&filter=qu=3;sl=9;minle=115
At one point the armor values of items seemed to decrease, but in a subsequent patch they returned to or exceeded their original values: [Dryad's Wrist Bindings] returned to normal but [Robes of Insight] gained 10 armor. The reasons for these changes are not yet known.
Sockets
A socket is counted as the stat values of an "expected gem".[2] The expected gem is believed to always be a blue-quality gem that matches the socket and, in the case of a colored socket, usually shares both a primary and a secondary stat with the item.
A meta socket on a Cataclysm item appears to be worth 40 points in the primary stat and 20 points in each of two secondary stats, which would make it equivalent to a yellow socket plus a blue socket. For example, compare 
[Helm of the Typhonic Beast] to 
[Stormforged Helm].
Free prismatic socket on relics
All relics have a prismatic socket that appears to be free when compared to a wand (e.g. 
[Relic of Norgannon] versus 
[Theresa's Booklight]).
Socket bonuses
Socket bonuses are proportional in value to the number of sockets, don't count toward the stat budget, and may or may not be a stat already present on the item. For Cataclysm items, the socket bonus is 10 points for a colored socket and 20 points for a meta socket. Prismatic and cogwheel sockets don't grant or increase a socket bonus.
Set bonuses
Set bonuses don't count toward the item budget.
Cooldowns and procs
Cooldowns and procs that increase a stat are counted as the amount of the increase times the expected percentage uptime. On PvE trinkets, the value is the same for a cooldown as for a proc; for example, the 
[Core of Ripeness] cooldown gives 1926 Spirit for 20 seconds every 2 minutes (1/6 uptime), while the 
[Fall of Mortality] proc gives 1926 Spirit for 15 seconds about every 90 seconds (1/6 uptime). On PvP trinkets, however, a stat cooldown is counted as 20% more than its average amount; for example, 
[Bloodthirsty Gladiator's Insignia of Victory]'s proc gives 912 Strength with about 1/3 uptime, while 
[Bloodthirsty Gladiator's Badge of Victory]'s cooldown gives 1520 Strength (instead of the expected 1824) with 1/6 uptime.
The crowd-control-breaking PvP Trinket cooldown replaces a stat cooldown or proc. This means that as the item level increases, its cost increases but is a decreasing percentage of the budget.