Gurugram property hunt: Surprising costs and negotiation limits

Gurugram mein property ka 'all-in cost' nikalna is a full-time job, yaar. I'm currently in a 2BHK in Sector 82, thinking of upgrading to a 3BHK, maybe near Sector 109. Visited a few projects, including MRG World. Builders bolte hain price fixed hai, but does negotiation actually work here? Kitna percent realistically knock off kar sakte hain? Also, ye PLC, stamp duty, registration ke alawa aur kya hidden charges hote hain? Already have a home loan, don't want any surprises later. Koi experience share karo please.