Ancient Stone Idol's cost is reduced for each attacking creature, not just creatures attacking you. It even counts your attacking creatures.

If there are enough attacking creatures, Ancient Stone Idol's cost can be reduced to {0}.

The total cost to cast a spell is locked in before you pay that cost. For example, if you control five attacking creatures, including one you can sacrifice to add {C}, Ancient Stone Idol costs {5} to cast. Then you can sacrifice the creature when you activate mana abilities just before paying the cost, and it still costs only {5} to cast.