Is there a website or even Android app that keeps its subscribers updated of any possible promotions/discounts going on at Google Play (previously Android Market)? I hate to miss the train.
Your best bet would be to follow Google Play on Google+. They always post any kind of news, including promotions for apps, music, movies...
I use AppSales. It works well for me.
I've never used it, but this app seems to be what you were looking for: https://play.google.com/store/apps/details?id=com.echolu.android.apphunter
You can set up an alert for the Google Play store at http://dealnews.com. It will email or text you new deals come up.