2. Increased Efficiency of Mana Cost (Skill Efficiency)
This stat reduces the overall mana cost of your skills by increasing their efficiency. This is more of a hidden modifier but Path of exile currency can result in better mana scaling.

Example Mods:
"5–15% Increased Efficiency of Mana Costs"
"Skills have 10–20% Increased Mana Cost Efficiency"
These types of mods effectively make your mana go further by reducing the total cost of your skills in a more "passive" way, improving mana sustainability.

3. Passive Tree & Ascendancy Nodes
Some Ascendancy classes and passive tree nodes provide Skill Cost Reduction or increase mana efficiency in other ways. Here are a few examples:

Ascendancy Classes:
Hierophant (Ascendancy for Templars)

Divine Guidance: Reduces mana costs of skills and grants increased mana regeneration.
Divine Bloom: Provides reduced mana cost for all skills based on the amount of mana you have.
Occultist (Ascendancy for Witch)

Forbidden Power: Increases the effectiveness of your Mana Regeneration while reducing the mana costs of certain skills.
Passive Tree:
"5–10% Reduced Mana Cost of Skills" nodes are commonly found in the Shadow and Witch sections of the passive tree.
"Mana Efficiency" nodes often increase the effectiveness of the amount of POE currency for sale mana used, essentially reducing overall skill cost.