Bangalore property negotiation: my reality check and cost queries

Negotiation in Bangalore real estate feels like a myth sometimes, honestly. Builders always quote a fixed price, but I'm not convinced. Currently in a 2BHK in Whitefield, thinking of upgrading to a 3BHK, maybe near Sarjapur Road or a Prestige project. Already have a home loan, so har ek paisa counts. Log kehte hain ki 5-10% tak negotiate ho jaata hai, but is it true for a 1.5 Cr property? Ya fir bas small stuff pe? Also, all-in cost ka kya scene hai? Stamp duty, registration ke alawa kya hidden charges aate hain? PLC, parking toh pata hai, but aur kuch hota hai kya jo end mein shock deta hai? Want to avoid surprises. Any tips on how to haggle ya kaun se costs pe dhyan dena chahiye?