You did not mention which portion of the California Vehicle Code you were cited under but a seat belt infraction usually falls under VC section 27315. Likely you were cited for driving without a seat belt under 27315(d)(e) of the Vehicle Code which states the mandatory use of safety belts is required. Subsection h of this part of the California Vehicle Code states:
(h) Any violation of subdivision (d), (e), or (f) is an infraction punishable by a fine of not more than twenty dollars ($20) for a first offense, and a fine of not more than fifty dollars ($50) for each subsequent offense. In lieu of the fine and any penalty assessment or court costs, the court may order that a person convicted of a first offense attend a school for traffic violators or any other court-approved program in which the proper use of safety belts is demonstrated.
The fine listed for a ticket for the 27315 offense on the CA Uniform bail schedule is $76.00. This is for a first offense so as you can tell the $20 penalty grows a bit when you add on court costs, county fees, etc.
If this is your first offense for a seat belt offense you may see if you are allowed to take a traffic school class instead paying the fine, assessment and court costs.
Since January 1, 2004 the cost of a seat belt ticket has more than tripled, now costly generally from $76 to $89 for a first offense, depending upon the county. Legislation (AB 1625) from this time period allowed penalty assessments and court costs to be added on top of a base fine. In essence since 2004, what previously cost $22.50 on a first offense will now run more than three times that amount. Drivers will also face penalties if passengers are not safely buckled as you learned from the police officer.
If you were cited under a different portion of the California Vehicle Code you can search the laws here to see if it lists what the fine amount will be and also look at the Uniform Bail Schedule for CA.
If you were cited for a child being without their safety belt buckled the fine would be higher. Children who are required to be secured in safety seats (under 6 years old or weighing less than 60lbs) have to ride in the rear seat of a passenger vehicle. There are several exemptions, including cases where:
• there are other children in the back seat
• a restraint system cannot properly be installed
• there is no rear seat
• the rear seat is rear-facing or side-facing
Infants less than one year of age or weighing less than 20 pounds must ride in the back, no exceptions, if there is an active passenger air bag in the front seat. A first offense is punishable by a fine of $100; second offenses will cost $250 (Vehicle Code: section 27360.5).
For more information on your citation and fines contact the court listed on your ticket and you may also try contacting the CA DMV.