Registration of apartment

I am in the process of buying a apartment at Sarjapura- Atibele road. My concerns:
* I have been told by the builder to keep 30k cash handy for the registration. This is apparently to exepediate the process of registration at the sub registrar office in Sarjapura. I have also been told that the property value that would be in the Absolute Sales Deed would be the government prescribed value and not the actual value of the flat paid by me.
* Having spoken to many peoplw who have bought apartments here, I came to know that it is legal to undervalue the property. Is that so? Is it ethical to undervalue the property?
* I also need the name and contact details of the sub registrar at Sarjapur.