How profitable are doomsday/rapture myths?

They can be very profitable, indeed. You'd think that people would've gotten a clue after Y2K, but no. There are people who got seriously rich off of hustling Y2K paranoia, and they're probably right there ready to cash in on 2012, too. Rapture propaganda (such as the "Left Behind" series) has been a cash cow for years. The chances of marketing such things climbs even higher if the author is smart enough to add a few juicy conspiracy theories into the mix.