We are very sorry to hear about your mother's news and hope for a fast recovery.
Visitors medical insurance covers for any new, unexpected sicknesses and illnesses. This would cover things like: a snake bite while you are on a hike, a car accident, a high fever, etc. Since your mother is already diagonised with cancer, it becomes a pre-existing medical condition, which unfortunately is not covered by these travel medical insurance policies.
Any conditions that your mother is already aware of is considered a pre-existing condition. Whether or not a condition is considered pre-existing or not, is based on the doctor's discretion that you are seeing. Any second opinions or cancer treatments unfortunately would not be covered by a visitors insurance plan. For a comprehensive explanation of why, see: Pre-Existing Conditions and Visitors Insurance
However, we do offer plans that cover for the acute onset of pre-existing conditions. For more information, see: Acute Onset of Pre-Existing Conditions
Because health care is so costly in the US, it is best for your mother to seek healthcare in India.
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else