The most cost effective way to get a large library of Church Fathers is to order the Schaff set, aka Ante-Nicene Fathers and Nicene & Post-Nicene Fathers. Its 38-Volumes in total. You can get it for $299.99 for the whole set (about $8.50 or so a volume) in brand new hardcover at www.christianbook.com
Most, but not all of the Ancient Christian Writers series is included in that 38 volume set, and its the less expensive way to go. You can supplement with a few volumes of ACW, and the Popular Patristics series from Saint Vladimir's Press, which is also very reasonably priced (about $10 a volume).
I also highly, HIGHLY recommend to everyone who can find it the four volume set of Johannes Quasten's 'Patrology'. Its the ultimate reference guide to all of the Church Fathers, and the various published editions of their works. It includes summaries of all extant works at the time of its publication, as well as topical summaries of important doctrines and doctrinal areas by each of the Fathers. So, for example, if you're interested in finding out what St. Irenaeus had to say about the Theotokos or St. Gregory the Theologian's teaching on the Eucharist, its just a quick flip to get a summary, quotations, and references for further reading. The set goes for about a hundred dollars, and is worth every penny.