As mentioned in the original message on this post and the linked thread, some queries could allow reduced costs for both Kagi and the user.
The problem is that Kagi needs to be aware of these specific programming sites by having them crawled and indexed in an internal index and letting the user choose to use the internal index. Otherwise, this would likely trigger a search on external indexes and defeat the whole idea, which is not much different from the actual implementation of lenses.
The user needs a way to explicitly specify that the search should be done with "reduced quality" for a guaranteed reduced search cost. On the other hand, if the user only wants results from the MDN or other official documentation sources, then "reduced quality" is acceptable, and full index results are not necessary.
I posted the pros and cons of different approaches in a reply to the aforementioned thread.