California has a Uniform Bail and Penalty Schedule that recommends what fines and penalties should be handed out for various violations of the law however fines for traffic citations are jurisdictional. This means that judges in each county court can modify the fines for their specific jurisdiction.
Thus there is no way for us to quote a set amount for traffic citations statewide for California. You can get a ticket for the same offense in two different CA counties and the fines will be for a different amount. Be aware that your California car insurance rates will likely be affected since this is a serious violation.
However, the counties seem to stay close to the state schedule when determining fees. From our research we found that the minimum "suggested" fine for car-pool violations rose in 2007 to $380, So likely no matter the county the fine will be for around this amount. We have read that repeat offenders ticket amount can cost up to $1000.
To find out the exact amount of a carpool ticket in a certain jurisdiction, contact that county's court. If you have receive a carpool (HOV) citation then call the court listed on the back of the ticket to find out what your fine amount will be.