Zubrin, Mars Direct. He estimated $10-15b to do it, and if you were SpaceX, you could probably do it for less. The key to making Mars cheap is to do multiple launches -- first launch is a fuel production facility (automated) which uses Mars resources to make fuel and oxygen, then housing, then a return vehicle. Launch the crew once everything is set up, in a vehicle which can't return on its own -- they use the stuff already on Mars to live and return.
I mostly just made up the moon number, based on it being a whole lot easier than Mars. I think you could probably do a recreation of Apollo pretty cheaply (a quick flight to the moon, simple landing, etc.). Certainly a trip to the moon without landing wouldn't be THAT hard once you had a rocket able to escape from earth gravity (maybe 2x as hard as launching to GEO?). You could maybe do it with a Falcon 9 in a single launch, so that's about $2b. It's less fun if you don't land, though.
I mostly just made up the moon number, based on it being a whole lot easier than Mars. I think you could probably do a recreation of Apollo pretty cheaply (a quick flight to the moon, simple landing, etc.). Certainly a trip to the moon without landing wouldn't be THAT hard once you had a rocket able to escape from earth gravity (maybe 2x as hard as launching to GEO?). You could maybe do it with a Falcon 9 in a single launch, so that's about $2b. It's less fun if you don't land, though.