What's the real all-in cost for a 3BHK in Sohna?

The 'all-in cost' builders quote is never the final figure, especially in Sohna. I work in real estate, but buying for myself in Sohna, Sector 89A, for a 3BHK is a whole different ball game. Builders like SRV Landbase give one number, but I know there's more. What's the realistic percentage for stamp duty, registration, PLC, parking on top of the base price? Is 15-20% enough? And near possession, what other hidden charges usually pop up? Society formation, electricity, OC... kitna extra lag jaata hai? I'm trying to figure out if a construction-linked plan helps manage these better than a big down payment upfront. Anyone recently bought a 3BHK in Sohna? What was your true all-in cost experience like? Any pro tips to avoid surprise charges?