Depends on the project. Sometimes free options, link directories, link trades, etc, etc are enough, other times, it requires a full media blitz in the high $$,$$$ per month range.
I personally just spend on PPC, I don't really mess with any banner ads at all! I will try some in the future, I hear you can get some good results from them!
Until now, I spent less than a dollar in the advertising matter, as I done most of the blogs/websites advertising by my own. Thus, it's not only saving me lots of money, but I also have the chance to gain some ads related experience as well...