Builder demands extra ₹3 lakh for flat in Bangalore after agreement, is this legal?

I booked a flat in Bangalore last year and signed an agreement with the builder. Now, just before the registration, they are demanding an extra ₹3 lakh, saying construction costs have increased. I am really stressed and don't have the additional funds. I feel like they are extorting money from me. Can they legally do this after signing the agreement? What legal options do I have to fight this?