Under single player custom match settings is the Cheat Mode. It's only available in a custom single player match, I can't emphasis this enough.
Disabled - Default
Regular Items - Select a loadout of regular items to be equipped before a match.
Treasures - Select a loadout of treasure items to be equipped before a match.
Companions - Select a loadout of companion cards to be equipped before a match.
No Gold Cost - Cards cost no gold.
No Rot Cost - Cards do not increase Rot when you use them.
No Mana Cost - Cards cost no mana.
Enabled - All of the above.