Assuming there are 10 creatures swinging away at me, if I flash in Ancient Stone Idol paying 2, then another player wipes the field of creatures in response.
The question is:
Does my Idol still come in at the 2 drop or do I have to pay the additional 10 at this point to continue casting it?
In fact you pay 0 to cast Ancient Stone Idol in this case.
Ancient Stone Idol looks at the number of attacking creatures at the time you're casting it (C.R. 601.2f). Thus, if there are ten attacking creatures, Ancient Stone Idol costs 10 less to cast (so that the total cost is 0, not 2) (C.R. 601.2f). Once the cost is figured out, it's locked in and won't change even if the number of attacking creatures changes afterward (C.R. 601.2f).
Note that while you're casting a spell (such as Ancient Stone Idol), players don't have priority to cast other spells, including spells that would remove creatures from the battlefield (C.R. 601.2a-i have no priority window until the spell is cast).
The question is:
Does my Idol still come in at the 2 drop or do I have to pay the additional 10 at this point to continue casting it?
Ancient Stone Idol looks at the number of attacking creatures at the time you're casting it (C.R. 601.2f). Thus, if there are ten attacking creatures, Ancient Stone Idol costs 10 less to cast (so that the total cost is 0, not 2) (C.R. 601.2f). Once the cost is figured out, it's locked in and won't change even if the number of attacking creatures changes afterward (C.R. 601.2f).
Note that while you're casting a spell (such as Ancient Stone Idol), players don't have priority to cast other spells, including spells that would remove creatures from the battlefield (C.R. 601.2a-i have no priority window until the spell is cast).