The 50% MP reduction for the next spell is based on the original cost and is not affected by the 10% reduction from Dark Arts.
Conserve MP can still activate while Parsimony is in effect.
Savant's Pants +1 and +2 cause Conserve MP to take effect every time during usage of this stratagem.
The word parsimony means extreme or excessive economy or frugality, or stinginess. Unlike penury, which implies conservation out of necessity, parsimony implies conservation for the sake of prosperity.