Builder demanding ₹5 lakh extra to deliver flat, what are my rights?

I booked a flat in Pune with a reputed builder and paid 90% of the total cost. Now they are demanding an additional ₹5 lakh citing 'increased material costs' before handover, which was supposed to happen last month. I feel helpless and don't know if this is legal. They say they'll cancel my booking if I don't pay. What can I do to protect my investment? I am worried about losing my hard-earned money.