It's hardly surprising that the assassin's casting is confusing, the entire class is composed of nearly pure fail. I'd support the answer above, though why on earth they didn't just put the proper entry under weapon and armor proficiencies like every other class is a complete mystery.
As for the ability you're looking for, there are actually quite a few ways to do that.
Divine Metamagic feat - CD - uses Turn/Rebute Undead attempts to lower metamagic cost
Easy Metamagic feat - Dragon 325 - reduces cost of 1 metamagic feat by 1
Arcane Thesis feat - PHB2 - +2 CL & -1/level for each metamagic feat applied to selected spell
Practical Metamagic feat - Races of the Dragon - reduces cost by 1 for 1 metamagic feat, must be Dragonblood subtype and spontaneous spellcaster
Metamagic School feat - CM - requires Spell Focus (chosen school) or specialist wizard in chosen school - reduces the cost of adding metamagic feats to spells of a chosen school of magic by 1
Improved Metamagic feat [Epic] - DMG, Spellcraft 30 ranks needed, -1 to metamagic cost, stacks with itself