Well, I went and found the exact commands to use and scripts to put them in to do that, so here's my explanation:
Well, you'll want to first (like always) copy and paste a default Indirect Effect so as to make sure that the rest of the slots have the necessary Scripts/Events in them and that the Indirect Effect functions properly otherwise. We'll call that step one. Also, make sure it's a Condition type as opposed to an Action type Indirect Effect.
After that you'll need to make the Indirect Effect a) half the consumption of MP and b) allow Abilities to be casted even when party members have half (or less) the MP cost of an Ability. Those'll be steps two and three. These scripts already occur in the battle system and do not need to be placed anywhere (they're Default scripts).
First, this is the "lesson about Indirect Effects" I was going to give you. Let me now teach you about the Data/Target/Status/Action/Confirm and Data/Target/Status/Condition/Confirm script commands. This is how you will always set it up:
Target Data: Action Status Confirm [Poop] ......... (if the enemy is currently afflicted with the Action Status called Poop, Default Flag 170 Indirect Effect will be turned On)
Script Branch: Condition: Flag: [170 Indirect Effect] On ......... (and thus, you set up a script branch using Default Flag 170 Indirect Effect)
Text: Message: 'Currently afflicted with Poop!' ......... (if Default Flag 170 Indirect Effect is On, something happens... and you put that something here - your something will hopefully be different
)
Script: Condition End
Make sense? I'm sure you could imagine how useful that could be. One example (and by far the most common) would be an item/ability that checks to see if a member/enemy is afflicted with a specific Action Status and heals the member/enemy of their Action Status only if they're currently afflicted with that Action Status (yes, this is the only way to do it), and I'm sure there are a thousand more (such as this gem).
Anywhoo, I'm sure you remembered that our two steps were to a) half the consumption of MP and b) allow Abilities to be casted even when party members have half (or less) the MP cost of an Ability. Well here's a) half the consumption of MP:
Default Script (roughly #41)_Consume MP 1 .........
Delete the whole thing and put in its place:
Data: Load Ability Info
Battle: Substitute Target Attribute for Variable
Data: Variable: TempVar0 = Var156 Ability:Cost + 0
Target Data: Condition Status Confirm [Poop]
Script Branch: Condition: Flag: [170 Indirect Effect] On
Data: Variable: TempVar0 = TempVar0 / 2
Condition End
Data: Variable: Var98 Magic Points = Var98 Magic Points - TempVar0
Battle: Substitute Variable for Target Attribute
And here's b) allow Abilities to be casted even when party members have half (or less) the MP cost of an Ability:
Default Script (roughly #60)_Start MP .........
Delete lines 0-7 and put in their place:
Data: Flag: [No Instg Action] Off
Data: Load Ability Info
Battle: Substitute Target Attribute for Variable
Data: Variable: TempVar0 = Var156 Ability:Cost + 0
Target Data: Condition Status Confirm [Poop]
Script Branch: Condition: Flag: [170 Indirect Effect] On
Data: Variable: TempVar0 = TempVar0 / 2
Condition End
Script Branch: Condition: Variable: [Var86 Magic Points] >= TempVar0
Data: Flag: [No Instg Action] On
Condition End
Now show us a game, or at least a demo!