Hidden costs beyond the flat price are insane

The 'all-inclusive' price builders quote is such a scam. I'm a first-time buyer, looking at properties in Sector 95A, like MRG World's projects. Honestly, I thought the price per sqft would cover most things. But now I'm seeing so many extra charges – stamp duty, registration, PLC, IDC, IFMS... it's overwhelming. My budget for the flat itself is around ₹70-75L, but these add-ons are pushing it way higher. My parents keep saying factor in at least 15-20% extra, and I had no idea it was that much. How much did you guys actually end up paying beyond the base price? What are the biggest hidden costs I should watch out for? Should I negotiate these or are they fixed?