In the adult Sunday School class that I teach, we have been using as our texts the small books written by N.T. Wright in the Paul for Everyone series, following the lead of Macon and Walter when they taught their Sunday School class at West Lake. They started with Wright's study on Romans, and we followed, then to the Gospel of John, and now to Galatians. With each of these studies, I used other commentaries and texts. I have been able to acquire most of these other books in used editions at a reasonable cost through either half.com and abebooks.com. I buy the N.T. Wright texts new from Amazon, however. At the end of a study, most of the students will give the Wright texts back to me, and I put them on half.com and recover about 40% of their cost.
To supplement the Galatians study, I am using Stott's The Message of Galatians in The Bible Speaks Today series published by Intervarsity Press. I would use a Stott study as the main text, in preference to Wright's, in every case, but the IVP editions are more expensive than the Wright texts, and many of the people in the class pay for their class text. Furthermore, Stott doesn't necessarily have a study for each book in the NT.
I am also using Luther's Commentary of the Epistle to the Galatians, which, according to Stott who quotes often from this commentary, is based on lectures delivered by Luther in 1531. Amazon publishes a Kindle edition of this commentary and charges nothing for it. I also have John Brown's An Exposition of Galatians, which Stott also cites from time to time. John Brown was a Scottish minister, born in 1784, who "entered the University of Edinburgh at the age of 13, and in three session in the Arts faculty, gained such a command of Latin as to read it in like his mother tongue," according to the introduction in the reprint edition of this commentary published by Sovereign Grace Publishers in 1970.