Using your library is the cheapest. If they don’t have the books, they can get them for you with “interlibrary loan”.
The next cheapest way is probably to buy them all as graphic novels — 10 (or 12, or 14) volumes, at about $20 each. The next cheapest is the two volume Omnibus edition, and after that, the oversized Absolute Editions.
I’ve just put the Absolute editions on my wish list. Now all I need are friends to buy them for me, or more overtime at work.