Many Kindle books now more expensive than hardbacks because both publishers and Amazon want you to buy books, where they make most money.
1. Publishers want to to get put off by expensive ebooks,and since libraries are always out them to lend, you got them to buy the paper version, which you can't pirate easily.
2. Amazon wants you to sell expensive ebooks because they make more money, since they don't have to pay that much royalty to authors.
Now, how can you find free ebooks on Amazon?
This lists all free Kindle books. From there, you can drill down into categories and use any of the left navigation bar filters.
This link just adds a search keyword capability to the first link. Click on the "search" part of the "results for" line and you can change the search term to whatever you desire.