Yes, costs average less than $6 a month. Wholesale SIP termination and origination is remarkably cheap if you are dealing with the US48 states and Canada.

I believe I'm at something like 6/10ths of 1 cent per minute.

Yes, that's about right. It's just a matter of whether you're willing to pay $6/month for this. :)

$6 a month is more than a fair price for me to eliminate robocalls; I receive about 5-10 on average in any given day.

The collateral damage are systems that have you mark your place in line and call you back. It seems like these could be manually whitelisted though with some ingenuity.