A plan will not cover sickness or injury that has already occured, as it would be considered pre-existing. Since pre-existing medical conditions are not covered by any of the short-term visitor insurance plans, buying insurance to seek medical treatment for existing medical conditions will not be helpful. It is advisable that you buy insurance prior to your trip.
Read more about insurance for pre-existing conditions, here: Pre-Existing Conditions for Visitor Insurance
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else