According to US copyright laws, all copyrightable works published in the United States before 1923 are in the public domain. A massive effort is underway to scan and catalogue these public domain works and provide them, free of charge, through the internet (see http://www.archive.org/index.php to search for your favorites).
I guess sometimes the best things in life are free.
Given that the final book of the New Testament was penned over 1900 years ago, most of what needs to be written about the sacred text, has been written about the text over and over throughout the last 1900 years. Although not dealing with the latest scholarship on any particular issue, about 95% of what a person needs to learn regarding any issue in theology or exegetical studies can be gleaned from these on-line libraries.
“The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.”
Ecc 1:9 (KJV)
For my forthcoming expositional study of the book of Galatians, I’ve been scouring the net for resources. See my Google Library for a sampling of useful works that can be studied for free.