First, in many California counties, it is common for the tax-assessed values to be far below the market values. Therefore, a discrepancy between the sales price and the tax-assessor's value is not evidence of fraud in and of itself, in my opinion.
However, the appraised value of your property should correspond with the market value closely. If evidence shows the appraiser and loan provider conspired to mislead the consumer and investors who provided the money for the loan, then that is the elephant in the room.
If you can show your county or city district attorney that the builder, an appraiser, and a mortgage lender or lenders had a scheme to overvalue property, sell it to unsuspecting consumers, and market the mortgages to unwitting investors, then any honest district attorney would love to present this criminal fraud case to a jury.
Similarly, if you can bring together several homeowners who were victims of a scheme to defraud them, any attorney with litigation skills and ambition would love to file a civil fraud lawsuit against the mortgage lenders. An attorney who believes you have a strong case will work on a contingency basis.
I would like to offer one note of caution in the form of stating the obvious. Some areas of California have experienced a relatively small dip in home values in recent years. Other areas have been devastated with property values dropping to half of what they were at the peak, and sometimes less. Just because the market value of your property dropped after your purchasing it in 2008 may not be evidence of anything other than you are in a neighborhood where residential values dropped across the board. I am not saying you have no case -- you may have an excellent case. I just want to remind you what has happened to the value of residential property since the peak in 2007.
I hope this information helps you Find. Learn & Save.