File an RTI (Right to Information) request for the "2001 Ready Reckoner for Mumbai district."
If you access a 2001 PDF today, the difference in figures is startling. Areas like Bandra, Andheri, or Lower Parel had RR rates that were a fraction of their current market values.
For any property purchased or inherited before April 1, 2001, the Income Tax Act allows the owner to use the Fair Market Value (FMV) as of , as their cost of acquisition.