I cross the border frequently. I am asked all the time about currency. I have been stopped by CBP going south, asked about money (last month Eagle Pass) and had them count my money.
There are 2 sets of laws in play. You must declare to Mexico and you must declare to the USA. Mexico has confiscated money at airports, Cancun, for not reporting.
Why take the chance? It takes 5 minutes to comply.
BTW, if you are traveling as a "couple" once you hit $10,000 you must report it. It is not $10,000 each person