This post only required 13 billion RC. I guess the cost for a post/comment is about the same no matter how big they are.
EDIT:
Looks like edits are going to be a big problem. Simply changing the title of my post cost the same amount of mana as creating a whole new post.
So happy I just read that comment because i edit comments a lot, and although I've been trying to track my mana usage over the last hour or so I doubt I would have thought to check for edits. It's frustrating as shit, but like you said, I think in the long run it will work out in our benefit