What's the real deal with negotiation and hidden costs?

Is negotiating on property prices in Noida Extension even a thing? I'm currently in a 2BHK and thinking of upgrading to a 3BHK, maybe around Sector 149. My budget is around ₹1.2 Cr, but builders often quote higher. How much can you realistically get them to budge? I already have a home loan, so every lakh counts. What's the true all-in cost people are seeing? Beyond base price, stamp duty, registration, and PLC, are there big hidden charges popping up just before possession? Heard stories about maintenance advances, club membership, IFMS... all adding up. Should I factor in an extra 5-7% for these surprises? What's your experience been?