Page 8 of this NASA document is the most trustworthy single page source I've found so far: http://www.nianet.org/ODM/presentations/Overview%20SVO%20Ken...

This chart gives (among other things) fatalities per passenger mile for 2005-2012 as a multiplier of the per passenger risk of death when traveling by automobile.

  Passenger auto:     1x
  Commercial Airline: .006x
  Motorcycle:         40.2x
  General Aviation:   12.0x

This would suggest that if all else is equal, and you are scheduling a trip from point A to point B, your chances of dying are about 1000x worse if you travel by private plane than by commercial airline, about 10x worse if you travel by private plane rather than driving, and that it would be about 4x safer to take a private plane rather than riding a motorcycle to your destination.

Of course, all else is not necessarily equal. A daytime motorcycle ride in full gear in good weather is definitely safer than an unexpected night flight in the fog with a low hour private pilot. A trip on a well-maintained corporate jet with an experienced pilot is almost certainly safer than snowy roads in a rear-wheel drive automatic. And of course, statistics don't answer the question of what should and should not be acceptable risks.