Leon,
I would look to the beckett almanac for the base price. However, being in the trenches looking for these cards, I can tell you that most collectors put a premium on the cards depicting players of the West Virginia schools (Marshall College and West Virginia University). University of Kentucky and University of Louisville, as these were the "big schools" in the area where the gas stations were located which gave the cards away, command about $25 to $50 (excluding Adolph Rupp).
There was a Murray State College in the set, and those cards are pretty rare, as the Kentucky schools go.
The best article which I have read that cover these cards is from a May 2004 issue of Beckett basketball. Below is a link to that article.
http://www.jeffreymullen.com/publications.htm
Also, supposedly the Aetna Oil backs are more rare than the Ashland Oil backs. However, I have seen plenty of both for the Kentucky schools.
Hope this helps.
Phil