http://www.twilio.com/pricing-signup says that it's 3 cents/SMS - which, yes, works out to be $3 - but not because it's per minute.

My point is that you have 86,400 seconds in a day. Well before you are spending $864 per day maxing out the number of messages you can send, you have $1 in the bank to get another number.

But then again, having a bank of 100 phone numbers just to send the million messages you want to send within an hour sounds slightly annoying from a logistics perspective.