How to effectively delete Google App engine Search API index

I have found some similary questions here ,but no solid answer.

• How to delete or reset a search index in Appengine
• how to delete search index in GAE Search API
• How to delete a search index on the App Engine using Go?
• How to delete a search Index itself

I see some googler suggest that

You can effectively delete an index by first using index.delete() to remove all of the documents from an index, and then using index.delete_schema() to remove the type mappings from the index 1.

Unfortunately, golang sdk does not have "index.delete_schema()" api. I can only delete document one by one by getting itemId list from index. And We got a surprisely billing status in dashboard:

Resource                       Usage           Billable         Price              Cost
Search API Simple Searches  214,748.49 10K Ops  214,748.39  $0.625 / 10K Ops    $134,217.74 

Can someone tell me how to effectively delete Google App engine Search API index wihout cost so much ?

Unfortunately there is no simple operation that allows you to delete an entire large search index without incurring substantial cost, short of deleting the entire app (which, actually, could be an effective approach in certain circumstances).