In Louisiana as in all other states you need to keep your registration and license plates up-to-date and not let them expire. Louisiana Revised Statute (LRS) 32:51 states that a vehicle license is required. It goes on to say that no person shall operate, or permit to be operated, any motor vehicle upon the highways of this state unless it is registered with the commissioner, the license tax is paid thereon, and it is operated in accordance with the provisions of this Chapter and other laws of this state.
As for the fine amount associated with the offense of driving around with an expired plate in Louisiana it appears to differ according to the county in which you were cited. For example the Sulphur City, LA court lists an expired license plate fine at $132. The Baton Rouge city court lists an expired license plate violation as a $97 fine and we have heard of drivers that have paid up to $200 for a conviction of this offense. Whatever the base fine is, remember that normally there are court fees, administrative fees, etc that will also be added to this amount.
We have also read that in LA a car can be impounded for 30 days for certain offenses, such as driver's license violations, selected parking violations and registration tags expired over 6 months. So if you have not renewed your plates within 6 months of the expiration date then your car could be impounded by law enforcement.
If you were given a ticket for driving with expired license plates and what to find out the exact fine amount, call the court listed on the citation. You may try asking the clerk of the court if the violation can be reduced or dismissed if you get your registration / license plates renewed before the court date and bring in proof.