On the point of costs: Yes I like that the show clearly established some lines where magic can't be abused without real consequences. And what is interesting is that it seems to occur on both the mundane and extraordinary levels, depending on what degree the abuse occurs. Prolonged use of magic and powerful magics always comes with their consequences, and I can't recall examples from either show anymore but I think even just using simple magics will have very small consequences too. Regardless, that's how I prefer to look at it.
On Mr. Pole's point (and my point earlier) I really think that BtVS should have more clearly established a sense of difficulty with the practice of magics, especially magic of real consequence. They say that sort of thing from time to time (Giles and others saying things like "these magics are extremely difficult" etc. etc.) but it's just lip-service when after the fact all of those magics seem to be accomplished with ease. For just one example, the trio manage to perform powerful and difficult magic spells but are they supposed to be magically gifted/talented at all?!? I don't know, maybe they were supposed to be real wunderkinds of their own but really, the overall impression of magic from the shows is that it was generally too easy to do. The trio felt like your ordinary nerds on a power trip...setting to one side the deus ex machina of the absurdly advanced technologies they employed, how are they also supposed to be able to accomplish rather complicated and powerful magic of their own? It just seems like anyone who wants to can do magic of significance. The only prerequisite seemed to be a desire to be good at it, and the chief difficulty being finding the ingredients. Is that not the feeling anyone else gets from the show?
Couldn't they have shown more frequently efforts to accomplish magic failing? Like literally, the magic spell fails to be accomplished (even by more powerful magicians like late Willow or Giles etc.) and the protagonists have to find another means. Although I can't remember, that probably has happened on the show, but in my opinion not often enough. I feel like the magic issue could have been tied up a little better than it was. Like other people said, it was left vague on some points and questions inevitably begin to emerge, like the questions raised in this thread.