There are initiation fees, ongoing membership fees, and local chapter fees.
http://www.math.binghamton.edu/alex/phibetakappa.html
While the fees may not seem like much to those of us working, some college students have pretty hard choices to make with their money. Like whether they get to eat and/or stay warm. While you may not have noticed them, there ARE homeless students. Often they find a place to hole up in campus buildings, or under them.
Obviously no one is going to decline acceptance of Phi Beta Kappa if there is no money involved.
Ken300D