Home buying & Black money

In the process of buying a property. Builder asking for 25% money in black. After talking around, learnt this has been going on for years and years and has now become an integral part of buying a property. The real reason we can never know the real cost of buying a property if because of the black component involved. Ready Reckoner rates, Agreement value and every other metrics stop making sense as soon as the black component is involved. The system needs to change, and I guess it never will.