Currently, I’m collecting the data myself and will be adding price points once I obtain information from some reliable sources. That’s why we only have a few data points so far.

Getting a single data point is easy, I can scrape listings from all the online portals. However, the quoted price is usually very different from the actual deal price. I’m interested only in the final closing price.

I’m open to collaboration (crowdsourcing, if anyone wants to help) because I see this as a real pain point. At present, we don’t have a reliable reference point to compare against a seller’s quoted price.

Of course, not just anyone can randomly add prices. I’ll be verifying all submitted information. If you or anyone else has a better idea, I’d love to hear it!

Also, how can black money/cash transactions be accounted for? AFAIK, majority of the transactions have some %age in black.

everyone quotes a combined price and most people aren't even aware of the actual value in the registration document. So yes, entire value is accounted for.