The biggest reason is that the SERPs for the various keywords which you have ranked well before have fallen.
Wow, a lot of supposition here in this thread, but this pretty much says it all: If you're making $450/month in google ads, then stop complaining, you're doing better than (most) individuals are, and by doing no work at all. Blogging is not work, by any means.
smart price is what happens when google finds out that people click the ads on your blog but it does not convert well for the advertiser.As a result the low priced ads are shown on your blog;cpc decreases eventually earnings go down.