You should carry cash to have meals in Kathmandu and on the trek. To answer to unplanned expenses like Lukla flight schedule, bad weather, extra nights on trek, emergency rescue etc. For food you should keep about 25000 to 28000 INR. For other unplanned expenses, you should keep about 5000 to 10000 INR. Emergency rescues are very expensive and costs as high as 3500 USD sometimes. However, for this, you can keep your passport as a security and release it after arrangement of the payment due to the helicopter rescue agencies. If you buy insurance, you will be shielded from these unplanned expenses. Do keep some extra money for tipping the guides and porters. They usually expect that you will tip them after the trek completion.