When you cast a spell "for free" it costs 0 mana and requires no additional costs either. "For free" is synonymous with "for 0."
You must still meet the threshold requirements to cast the spell.
If you cast a spell that costs X "for free" and you can choose the value of X, e.g. , then X must equal zero. But, if the spell itself specifies the value of X, e.g. , then X = whatever is specified by the spell.