I've read in many places that the reason NCR money is so deflated in value is that the Bros managed to destroy/irradiate/render useless the NCR's gold bullion reserves.

... But I've never actually heard this in the game. Where is the source of this?

Never heard it in game before. I simply though the Ceasars legion's money was only worth more because it was gold, caps because it was metal and hard to come by. By contrast NCR's money is simply paper that wastlanders would not trust as much as the tried and true caps and the obviouly valuable gold.

The loading screen tells you the Brotherhood destroyed the NCR gold reserves and deflated their money. Also, the quarry leader in Sloan says something about the money being worthless due to no gold.

Edit: double post

Actually, you get a decent explanation from the people at Sloan. Here's a basic explanation. NCR had gold reserves, and was pegging the value of the money to it. In comes Caeser's Legion, which destroys said reserves and obtains it for their own. NCR has no choice and looks for an alternative, this being water (being as clean water is so hard to come by) but since water is of course less valuable than gold, the value of said NCR money drops. Meanwhile, using gold and silver buillion obtain through whatever means, the Legion also crafts it's own money.

NCR dollar just get crap exchange rate in New Vegas, as it is NCR trading is limited.

And it is BoS who surgically destroy a number of NCR gold mine.