Interesting suggestion. I am not sure about it costing the same karma. The net effect will then be, that the threshold of acquiring 'downvoting power' will shift further upwards. For example, people who get to 501 karma, will have to wait to get to decent threshold (perhaps 550) to feel comfortable down voting even when they want to.
May be, they could just limit the downvotes per user, to just 10 per day or something like that, so that nobody can abuse it.
On a different note, not related to your comment, a feature request (and saying it here itself, just to have a single comment on this page):
1) I have always wanted to have some basic information about HN users, as I hover over their handles. As of now, I need to click that link into a new tab to see that. It could be a useful feature to have, @dang & @scott, while you are at it.
2) Have a separate humor upvote. Often some comments (e.g. [1]) make you LOL. But you can't do anything other than upvoting them, as reddit style 'LOL' comments are not allowed. I think, we can do with encouraging some humor here, if it can be managed.
After all, this score system is called karma. And hurting someone else's Karma - even for valid reasons - is bad karma by definition :)