Builder refuses to register my flat unless I pay extra ₹3 lakh?

I booked a flat in Bangalore two years ago. We are at the final stage of registration, but now the builder is demanding an extra ₹3 lakh, saying the construction costs increased. He threatens to cancel the booking if I don't pay. I have already paid 90% of the amount. I'm feeling cheated and don't know if this is legal. What can I do to stop him from canceling my apartment booking?