And no, I'm not accusing you of doing any such thing. The idea is that if you're going to oppose something, you can pull any kind of big number out of your ass without being expected to justify it in any way.

Back in the 1980s when Reagan proposed SDI (it was never called "Star Wars" except by its critics), we routinely heard estimates of trillion dollar pricetags for the program. This was absurd because no decisions had been made on the types of technology required, the numbers needed, or anything else.

I remember some years ago when homeless advocates claimed there were several million homeless people in the US. When the Census Bureau made active efforts to count the homeless, the advocates went out of their way to discourage cooperation. After all, they knew the number was never that high and admitting it would hurt their agenda and especially their funding.

I also remember claims of some 40,000 children being abducted by strangers in the US each year. According to the FBI, the actual number is only a tiny fraction of that.

There was a vocal feminist who claimed some 50,000 women starved themselves to death due to eating disorders each year. The actual number was well under 100. Not 100,000. 100.

Going back to the moon will be expensive. That much is certain. Until the missions are defined, it's not possible to determine what technology will be needed, so it isn't even remotely possible to put a price tag on it. Going to Mars will also be expensive. Zubrin et. al. claim an estimate of some $20 billion, a number I find absurd based on the history of aerospace development. More likely, the number will be a few times that amount, but until the mission profiles are defined, it's impossible to be much more precise.

The converse of the Big Numbers lie is the knowingly small numbers lie. There is a strong tendency with government programs to low ball the initial price estimates to sell the program to Congress. Once the program is underway, it's hoped that either enough of a constituency will develop to continue supporting it no matter how much it costs (e.g. Medicare, Medicaid) or that they can sell the argument that they've already wasted tons of money, so why not spend tons more to finish the job? There are countless examples of this practice, such as the Reagan Federal Building in DC (final cost was $1 billion according to the news, about twice the original projection), the Big Dig in Boston (several hundred percent over budget), and numerous others. Off hand, the number of counter examples I can think of where an overpriced program was cancelled is pretty small. These include the Super Conducting Supercollider, the Sgt. York (DIVAD) anti-aircraft system, and a few other military systems.