The trip itself didn't cost "billions". A lot of that was research and development, building the launch facility, equipping "Houston" with all manner of equipment (the famous "Houston" as in "Houston, we have a problem")
You only have to buy a lot of that stuff once.
Here is the real argument:
You have commercial companies already taking people to the edge of space, or low earth orbit, for a reasonable fee for a rich person.
The biggest cost is escaping Earth's gravity. Once you're in orbit, it would just take a bit more propulsion to shoot towards the moon, and then you COAST there. It isn't like driving a car, where you quickly come to a stop due to friction. There is no air in space, hence virtually zero friction. You only need a bit of thrust to slow your speed when you arrive, to enter into lunar orbit.
And who said a rich person would go by himself to the moon? They would probably have to wait until they have 10-20 such people to make the trip economical.
But all that assumes that the Van Allen radiation belts aren't lethal to humans -- which they unfortunately are. That's why humans will always be stuck right here on Earth, right where God created and placed us. No Star Trek for us.