If you pay CBD $5 to become a "member" you should be able to get more than that in the savings on your first bigger purchase. You used to be able to get Calvin's commentaries for $99 if you were a member, while outsiders payed $150 or so. My membership has ran out, and I have no reason to renew right now, so I can't check to see if it is still valid.
I don't want to discourage anyone from owning a hardcopy of Calvin's Commentaries. I love them and will eventually purchase a set when it doesn't mean lugging them to Japan.
Nevertheless, for those who want to benefit this great resource and lack either the $$ or the space: http://www.ccel.org/c/calvin/
Each commentary is available in PDF and a variety of formats. When I'm studying a book I print the commentary out two pages to a side and double sided. It allows me to highlight it and mark it up for notes when I'm studying without worrying about marring a nice book.