I drove down to US from Canada and we were asked if we had any cash on us and my buddy had $6k on him and we declared it. Well that cost us about 2 hours of bullshit, they had the dogs go through our car they made us empty out our wallets and any bags we had with us.
I also know of a Canadian guy that was driving down from Canada, had $50k+ in cash and over $200k in jewelery he didn't declare any of it and they found it when they searched his car...he was arrested they took the cash the jewelery and his brand new 5 series BMW and he was not allowed back in the US for 5 years...
I think in the end he did manage to get the car back but lost the cash and jewelery