My own home in Ghaziabad: Builder's fixed price feels like a trap

Budget finalised, loan pre-approved – ab Dasna mein AIMS MAX GARDENIA DEVELOPERS ka project dekh raha hoon. I work in real estate, but buying for myself is different yaar. Kitna negotiation possible hai Ghaziabad mein? Builder bolte hain fixed price, but I know better. Aur possession ke time jo hidden charges aate hain, unse kaise bachein? Koi idea hai kitna total out-of-pocket jaata hai beyond agreement value?