Greater Noida mein property: Stamp duty aur final cost ka kya scene hai?

Since last year, kids have moved out, so I'm looking to sell our big house and get a smaller, manageable 2BHK in Greater Noida. Dekh raha hoon Sector 155 mein Sunshine Infrawell ke projects. Meri main concern hai true all-in cost. Builder price toh theek hai, but stamp duty, registration, aur hidden charges kitne add ho jaate hain? Kya secondary market mein deal better milti hai to avoid some of these? Ya fir builder se hi negotiate karna chahiye for a flat final price? Thoda guidance chahiye, please.