Crossed into Mexico last Saturday
Before my trip I went online and filled out an FMM and printed it out. I walked up to the border at PedEast and there was no one in either line, Nationals or Foreigners. As I walked in through the Foreigners line, a guy came out of the back office and waved me forward. I handed him my passport and the FMM. He stamped my passport, tore off the "Entrada" side of the FMM and handed me the "Salida" side. Then he waved me through. No questions, no payment.
I was ready to pay the $30. It pays to be prepared.