Noida negotiation is a sham, hidden costs kill budget

Budget finalised, loan pre-approved – ab Sector 135 mein 3BHK dekh raha hoon. Currently in a 2BHK, thinking of upgrading. Vardhman Group ke projects dekh raha tha. Builders bolte hain no negotiation, par kya sach mein? All-in cost mein kitna hide karte hain? Possession ke time kya extra charge karte hain? Any real experiences?