One of the easiest things you can do to reduce costs of leads and get better leads is to engage with your own post and the comments others leave on your post.
Engagement (i.e. comments) on an ad indicates to Facebook that 'people like the ad' so they start showing it for less money. On top of that, intelligent and informative replies to comments helps answer questions that people may already be thinking and it's not uncommon to get leads off the post commenters because you can ALSO message them directly, which I recommend.

What to do: Check your comments daily and try to respond as quickly as possible. (You only have 7 days in which you can directly message a post commenter). Do whatever you can do encourage comments to your comments to. You could ask a question at the end of your reply to encourage this. But just the act alone of commenting on your comments will 'fan the fire' of comments and usually the posts really light up with more comments.

Also, don't be afraid or concerned about dumb comments and delete them. Give accurate and informative replies and keep as many 'negative' comments as possible unless it's just outright spam or similar.

Another good idea is to ask for customer reviews but instead of putting them on some website, have them leave it as a comment on your ad.  This has shown to be tremendously powerful.  You just need to share the post link.

Below are screenshots on the process and here's a full walkthrough on what to do in video format: https://youtu.be/uPtcdRxdnvs