I strongly suggest checkin out Beempy.com if you want a close estimation as to how many votes/comments or transactions you can do with your effective Steem Power.
I took a quick look on your account and here ya go:
And NO! The cost depends on a number of reasons like what type of transaction it is and how long or the size of the transaction is. A long comment costs more than a small one word comment.
You can look up the cost of every transaction here: https://beempy.com/resource_costs
Well that's a pretty good question you got there and I can tell you that it's the same price for the two of them.
If you look at your profile page on steemit you'll see 3 counters at the top, one for followers, following and posts. Well it shouldn't suprise you that your posts probably a lot more that what it should be and that's because posts and comments are all considered as posts.
As such, it stands to reason that since they are considered as the same thing and the only difference between them essentially is that one can be resteemed and the other cannot, their cost in resource credits should be the same or at the very least similar.
It costs about 1.56 billion mana to make a single comment and It costs the same thing to make a post as well, I asked a friend who's mana was at 100% to help me check and she did. So I was able to calculate everything using the values I saw on steemd.com.
I hope this helps.