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

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.

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