Huge drop in adsense CPC

it is summer time - wait for winter

Your CPC is decided by the competition in your niche and the number of advertisers interested to place ads in your site. Without knowing the details of your website, niche, current CTR, CPC, RPM etc nothing can be said conclusively. If your niche is related to season then only it will change according to the season. Suppose your niche is closely related to christmas then you will high cpc and income during the season. I have done google adsense optimization service and increased the earnings of my clients in elance. If interested contact me by PM.

Those putting Adsense on their site need to understand how things work on the other side of the coin, the advertiser side.

Advertisers want to spend the least amount possible. They can do this by reducing their bid but also, improving their campaigns. Google Adwords (and many other PPC ad networks) has something they call Quality Score. In essence, it rewards advertisers that have good ads with better positioning and lower CPC.

Quality Score's main factor is the ad's click rate. If you do the math, you'll realize that Google (and you as a publisher) will make more money having ads with higher click rates, despite lower CPC.

In a way, you should be happy since it may be a sign that advertisers are making better ads (or poor advertisers dropping out). In any case, the system is in constant flux and there's nothing you can do about it. But average CPC is not the metric you should be looking at. Instead, are your click rates going up? If so, better ads means more clicks and more revenues for you, even if CPC goes down.