After kids left, buying a smaller flat

My current house feels too big now that kids have moved out, so I'm looking for a manageable 2BHK in Greater Noida. Thinking about Sector 142. Nimbus Realty jaise builders ke saath price negotiation kitna work karta hai? Log bolte hain kuch hidden costs bhi hote hain. Kaise pata chale total all-in cost kya hogi, yaar? Koi experience hai toh share karo please.