Is there a limit to how much cash one can transport within the US? I just wonder in case of the odd secondary screeing or so? I know internationally you're supposed to have less than 10K or declare it or something like that. Any such rules within the US?
There's no limit if you're not leaving the country, but if you have "too much," as subjectively defined on the spot, your best case is to be questioned and sent on, and your worst case is to have it confiscated under drug seizure and forfeiture laws (and if the money is confiscated, you may or may not be able to get it back after lengthy and expensive legal proceedings).
In other words, fly with cash at your own risk.
Nota bene: The views expressed in this post are not necessarily mine.
I flew last month (intra-US) with a very large amount of cash (well over 10K) with no problem, no secondary screening. I didn't tell anybody I was carrying it though. The cash was for a vehicle and the seller offered a reduced price for cash on the spot.
It did feel like I was doing something illegal just holding that much $$$.
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Depending on the jurisdiction you're flying though, local cops could get very trigger happy with the asset forfeiture laws and confiscate any amount of cash they arbitrarily decide represents 'ill gotten gains'.
Cops have been known to make side deals with airport screeners (this pre-dates the TSA) to call them when drugs or cash are found - they get a reward for turning you in.
Depending on your appearance, cops may decide to stop you and search your bag for no reason - and confiscate any cash found within.
A number of years ago, 60 Minutes detailed the bad luck of an elderly black man traveling through a southern airport with about 10K in cash - his life savings which he was carrying for a legitimate reason. He was stopped because of his appearance, subjected to a search of his bag, and the cash was taken away.
Best advise: conceal it well, don't let the bag out of your sight, and don't permit the bag to be searched by police without a warrant (except when you're going through the screening process).
If a withdrawl of $10,000 or more is made in cash from a financial institution, it must file a copy of FINCEN Form 104 (formerly Form 4789). This form and its instructions are at http://www.irs.gov/pub/irs-pdf/ffc104.pdf
If you plan on carrying $10,000 or more through an airport, it MIGHT be useful to have a copy of this filled out form, and perhaps a letter from your bank.
Oh, my. I think this thread is going to get lively!
Oh, this actually happened... at least once that I can recall. I may get some of the details a bit off, but IIRC, a woman traveled to Texas with something like $15k or so in her bag. The TSA called over a LEO, who decided she had to be a drug courier and confiscated the cash. She had claimed she was traveling to get a breast enhancement and wanted to pay in cash.
Now, she probably was a courier, but it just goes to show how unsafe traveling with large amounts of cash can be. Not only are you subject to traditional thieves, but also to the government seizing your cash and making you prove to them it isn't drug-related to get it back (which in courtrooms can be quite difficult--and quite costly!) And remember: when it comes to forfeiture cases, your money is assumed to be "guilty" simply by nature of its existence, unlike (most) criminal cases in which the defendent is presumed innocent.
That case above was entirely domestic in nature. There are countless other cases of the government seizing cash at airports (such as that J-pop artist's mom, who fortunately did win in court--she had the resources to fight it and win, unlike many people).