You can purchase the visitor insurance any time before person travel or after the person reach to the destination country. So if the person already reached US, you an still purchase but the start date can be as early as the next day of the purchase date. However, it is important to note that IF the problem already exist before the policy start, that particular problem or related treatment may not be covered as it can be considered as "Pre existing" problem. That's why it is recommended to make the purchase before the person start the travel. But plan will cover for any other future risk as per the policy.
To learn more what it covers and what it wont please visit: Visitors Medical Insurance To get Quote and make purchase online: Visitors Insurance Plans To learn more about pre existing conditions: Pre-existing medical conditions for Visitors Insurance
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else