Two passengers for the $680 pesos. Additional passengers $60 pesos. What does it cost to get 3 people across on the people ferry?Agreed. But your call.
The $ sign is for MXN pesos in Mexico, but I'd still not take a car across the channel. I see some crossings limit passenger cars to two occupants as well. And don't expect English translations on signs, anywhere.
Be forewarned: Anything you leave visible in a parked car will be stolen, and you will be responsible for damages to the car. I hope that you are taking good trip insurance that covers medical since your home medical probably is no good out of the US, includes repatriations, and includes coverages for your belongings, but it's best to avoid losses. For any car you rent anywhere, take full insurance coverage from the rental company.
For spending, it's best to acquire pesos at a bank owned ATM, but that's another story. Have a great trip.
Edit:Transcaribe is more expensive