I pondered this recently, and it seems to top out at a couple bucks per shot.

The problem is that the phone number tends to need to be persistent for the sake of security. You can't typically sign up for something that requires a phone number and then expect to be able to keep the account safe without maintaining exclusive access to that number.

I'm sure if it were cost effective, one of the password managers would have some kind of SMS integration, like Apple's hide my email, but for phone numbers.