As to economics of robocalls,
this NPR podcast cites robocall cost as 28 calls per penny. They go on to say how money can be made of the 0.5% of answered calls.
But even the fact a person answered a robocall is monetized; the confirmation that the phone number is valid and a person might answer is something that can be sold. As such, the
#1 FCC recommendation is to
not answer a robocall.
Interestingly, over the last two years, the FCC has implemented
new call authentication in attempts to reduce illegal robocalls.