Yesturday 28 clicks= $40 | Today 8 clicks = $0.74 -How does that work?

Cost-per-click depends also on the ads showing on your pages. Maybe that time, high-paying ads is the one who is most clicked.

 

Depend of your ads which show in difference country zone. The best value click almost from USA.

 

It is entirely variable and depends directly on the amount advertisers are willing to spend.

 

Different ads based on different keywords will pay less or more. There is no actual way of knowing.

 

To be fair, Google can change their bid or how much they want to pay you through adsense quite easily. Just keep watching it closely.

 

it will be a question of what niche you are in

 

Perhaps your clicks aren't converting for the advertiser, so the value of your clicks has been reduced accordingly.

Hard to say given you've only given 2 days statistics. Balance it out at the end of the month, and take things from there. Everyones earnings fluctuate a lot, not just yours.

 

Probably the visitors clicking on the ads come from US/Europe one day, and the next day, those who click on the ads come from Africa.

 

Maybe these clicks from different Countries.