Crossing from La Paz to mainland Mexico requires an auto permit with a real deposit. This changed in May 2011 from the old simple recording of your credit card. You need to apply and pay at the Banjercito office in Pichilingue. The deposit will be returned once the vehicle and permit are returned at the border. If you do not return your vehicle and the permit by the time your permit expires, Banjercito will keep the deposit.
Auto deposit fees are:
•2007 and newer models, USD $400.00 deposit
•2001 - 2006 models, USD $300.00 deposit
•models previous to 2000, USD $200.00 deposit
Pichilingue Port offices where you will find Banjercito
Reclaiming your deposit can only be done at the border when you are leaving Mexico. Here is a quote from the Banjercito web site
5. Return to your country of origin. Please make sure to register the return of your vehicle when crossing the border.
Make sure to visit any BANJERCITO module located at the border crossing when you are leaving Mexico. Present your vehicle and the Temporary Import Permit in order to register the vehicle as having returned to the country of origin. BANJERCITO will issue a return certificate.
It is very important that you complete the vehicle return operation. If you don't complete this step, Mexican authorities will assume that your vehicle remains in Mexico, which can cause problems in the future. The vehicle is considered returned, when you physically present the vehicle at the border module and the authorities validate the return from Mexico.