This article is from the Anime FAQ, by Steve Pearl with numerous contributions by others.
If you purchase your anime magazines at a Japanese bookstore like Kinokuniya or Nikaku, then you are getting your magazine for the best price short of picking it up in Japan. These bookstores usually charge $1.70 per Y100, which seems to be the standard rate for all Japanese periodicals. (This amount varies with the exchange rate) If, however, you are purchasing these magazines at a Comic shop, you are likely spending upwards of $15 each (with the risk that an unscrupulous shop owner might be removing the neat inserts). The reason why Comic shops are much more expensive is that Comic shops usually receive comic items at a hefty discount (around 40%). But they are getting these magazines at close to the same cost we are paying for them. So they have to mark up the cost in order to maintain the same margins.
Typical magazine costs:
In Japan: At Kinokuniya/Nikaku At a comic shop: $4.00 $6.00 $13
For the addresses of Nikaku and other Japanese bookstores, please refer to the Anime Resources FAQ.