The Gold efficiency of an item tells you how much stats it provides for its price (gold cost). This is a theoretical value with no direct game impact, used mainly to compare items.
A base gold value for each stat has to be defined in order to calculate gold efficiency. This is a form of "theory-crafting" because gold value is not a part of the game's rules; the number has to be derived from the prices of in-game items (see below).
Gold efficiency is then the ratio of the item's theoretical gold value to its actual in-game price.
In general the theoretical gold efficiency is most useful for comparing items' relative, theoretical values. The "real" value of an item during an actual game is influenced by many other factors that are not easily modeled (in terms of gold or otherwise).
Calculating Gold Efficiency
The formula for the gold efficiency of an item (as a percentage figure) is simple:
$ \pagecolor{Black}\color{White}{\rm Gold\ efficiency}={{\rm Gold\ value} \over {\rm Item\ price}} \times 100 $
The difficulty is in determining the gold value given the stats on the item. It is done by using the price of "reference items" (see next section).
If the item has passive or active effects they are only taken into account if they provide stats with a gold value (e.g. if an item's effect added a temporary flat movement speed, the gold value of this effect could be determined, because there is a reference value for movement speed). See the next section for how the base gold value of various stats is derived from "reference items".
The item price is simply the price from the in-game shop.
Since gold efficiency is usually given as a percentage value, we have to multiply the ratio by 100.
An item is said to be gold efficient if its gold efficiency is at least 100%. Usually, items that are less than 100% gold efficient have other (passive or active) effects that are not taken into account when determining their gold value. This then results in a theoretical gold deficit in the item's value; in those cases, the items passive or active effects usually make up for the deficit.
Base Statistic Prices
The 
gold values assigned to stats are not measures of the effectiveness of those stats. They are simply base values meant to be used in determining the theoretical value of other items (or effects that are not item-related, such as Dragon buffs or Runes).
A stat's gold value is derived from the least expensive basic item that provides only that stat:
| Champion Statistic | Base Item | Item Cost | Stat Raised | Gold / Stat Bonus
|
|---|---|---|---|---|
| Attack damage |  Long Sword
| 350
| 10 AD | 35
|
| Ability power |  Amplifying Tome
| 435
| 20 AP | 21.75
|
| Armor |  Cloth Armor
| 300
| 15 Armor | 20
|
| Magic resistance |  Null-Magic Mantle
| 450
| 25 MR | 18
|
| Health |  Ruby Crystal
| 400
| 150 HP | 2.67
|
| Mana |  Sapphire Crystal
| 350
| 250 MP | 1.4
|
| Health regeneration |  Rejuvenation Bead
| 150
| 50% HP5 | 3
|
| Mana regeneration |  Faerie Charm
| 125
| 25% MP5 | 5
|
| Critical strike chance |  Cloak of Agility
| 800
| 20% Crit | 40
|
| Attack speed |  Dagger
| 300
| 12% AS | 25
|
| Flat movement speed |  Boots of Speed
| 300
| 25 MS | 12
|
Life steal, lethality, magic penetration, on-hit damage, cooldown reduction, percent movement speed, and heal and shield power are not found isolated on any single item, so their gold values are calculated using the least expensive items that provide the stat: 
Vampiric Scepter, 
Serrated Dirk, 
Sorcerer's Shoes, 
Recurve Bow, 
Kindlegem, 
Aether Wisp and 
Forbidden Idol for each of the aforementioned stats respectively. Spell vamp (on an item) is currently only found on the non-static passive Lifegrip on 
Maw of Malmortius, making its gold value incalculable.
| Champion Statistic | Base Item | Item Cost | Stat Raised | Gold / Stat Bonus
|
|---|---|---|---|---|
| Life steal | Vampiric Scepter
| 900
| 10% LS 15 AD | 37.5 35
|
| Lethality | Serrated Dirk
| 1100
| 10 Lethality 30 AD | 5 35
|
| Magic penetration | Sorcerer's Shoes
| 1100
| 18 MC Pen 45 MS | 31.11 12
|
| On-hit damage | Recurve Bow
| 1000
| 15 OH Dmg 25% AS | 25 25
|
| Cooldown reduction | Kindlegem
| 800
| 10% CDR 200 HP | 26.67 2.67
|
| Percent movement speed | Aether Wisp
| 850
| 5% MS 30 AP | 39.5 21.75
|
| Heal and shield power | Forbidden Idol
| 800
| 5% HS Power 50% Base MP5 10% CDR | 56.67 5 26.67
|
Item Statistics
Basic Tier Items
| Base Item | Item Cost | AD Value | Gold / AD
| Long Sword Equivalence
|
|---|---|---|---|---|
| Long Sword
| 350
| 10 | 35 | N/A |
 Pickaxe
| 875
| 25 | 35 | 100% Efficiency |
 B. F. Sword
| 1300
| 40 | 32.5 | 107.69% Efficiency |
| Base Item | Item Cost | AP Value | Gold / AP
| Amplifying Tome Equivalence
|
|---|---|---|---|---|
| Amplifying Tome
| 435
| 20 | 21.75 | N/A |
 Blasting Wand
| 850
| 40 | 21.25 | 102.35% Efficiency |
 Needlessly Large Rod
| 1250
| 60 | 20.83 | 104.4% Efficiency |
| Base Item | Item Cost | Armor Value | Gold / Armor
| Cloth Armor Equivalence
|
|---|---|---|---|---|
| Cloth Armor
| 300
| 15 | 20 | N/A |
 Chain Vest
| 800
| 40 | 20 | 100% Efficiency |
| Base Item | Item Cost | MR Value | Gold / MR
| Null-Magic Mantle Equivalence
|
|---|---|---|---|---|
| Null-Magic Mantle
| 450
| 25 | 18 | N/A |
 Negatron Cloak
| 720
| 40 | 18 | 100% Efficiency |
| Base Item | Item Cost | HP Value | Gold / HP
| Ruby Crystal Equivalence
|
|---|---|---|---|---|
| Ruby Crystal
| 400
| 150 | 2.67 | N/A |
 Giant's Belt
| 1000
| 380 | 2.63 | 101.33% Efficiency |
| Base Item | Item Cost | MP Value | Gold / MP
| Sapphire Crystal Equivalence
|
|---|---|---|---|---|
| Sapphire Crystal
| 350
| 250 | 1.4 | N/A |
| Base Item | Item Cost | Hp5 Value | Gold / Hp5
| Rejuvenation Bead Equivalence
|
|---|---|---|---|---|
| Rejuvenation Bead
| 150
| 50 | 3 | N/A |
| Base Item | Item Cost | Mp5 Value | Gold / Mp5
| Faerie Charm Equivalence
|
|---|---|---|---|---|
| Faerie Charm
| 125
| 25 | 5 | N/A |
| Base Item | Item Cost | Crit Value | Gold / Crit chance
| Cloak of Agility Equivalence
|
|---|---|---|---|---|
| Cloak of Agility
| 800
| 20 | 40 | N/A |
All items base efficiency
The following table displays all the item base gold efficiencies (without considering passives and actives). Full item gold efficiencies (with passives) can be found on the respective items' pages.
Trivia
- Spell vamp used be a statistic available on items. It was valued at
27.5 per percentage point before it was removed from Hextech Revolver. It is still available through Maw of Malmortius' passive.
- The former values for MP5 and HP5 were 60 and 36 respectively. Flat MP5 is available on Doran's Ring, and flat HP5 is still available on items like Warmog's Armor and Doran's Shield.
- The snowball item Mejai's Soulstealer begins with a very low gold efficiency, but upon reaching maximum stacks has one of the best possible in the game.
- Warmog's Armor has no gold efficiency cap with Sion's Soul Furnace or Cho'Gath's Feast because they infinitely increase their health, which will scale Health regeneration.
- A Rabadon's Deathcap also has no gold efficiency cap with Thresh's Damnation or Veigar's Phenomenal Evil Power.
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