Thought I negotiated well, but missed something big in Greater Noida

I thought I was a pretty good negotiator, but buying my flat in Sector 12, Noida Extension, made me realize main galat tha. KCC Contractors se deal karte time, I focused on the base price too much. Later pata chala, actual negotiation scope was on PLC, parking, aur even interiors. Builders often say 'no negotiation' on base price, but there's always room for extras. Kisi aur ne yeh experience kiya hai? Kitna percent tak adjust kar sakte hain total cost pe? Wish someone told me this before signing. Hope this helps someone.