How RAM usage effects battery drain?
I doubt that there is a direct correlation between battery consumption and RAM usage. The hardware doesn't know which RAM cells contain 'used' data and which not. So there can be no difference in battery consumption on that level.

But I think that one could say that unnecessarily killing Apps causes a few extra CPU cycles when those Apps have to be re-started and therefore "uses more battery".

And yes, you definitely should let go of your app-killing habit. In most cases, Android is capable of managing the system's resources quite well and you shouldn't bother with manually killing Apps. They definitely won't drain more battery if they just idle around and consume some RAM. Of course, if those Apps are constantly using the CPU, they will drain battery (but this is usually a sign of a bug within the App or a badly implemented App).