I believe the pf_ring stuff is free for .edu usage 
That was my impression as well (you know because that's what it says on their website). Unfortunately, the last time I tried to get a 10 Gb DNA license for my Intel X520 NIC, I wound up getting bounced to a Silicom rep who wanted $500/interface. I pointed out the discrepancy and was told that this *was* the educational price and that because the 10 Gb DNA driver development had been subsidized by Silicom, Luca had no choice in the matter. This was on March 7 of this year.
If you can get by without DNA (which provides a definite performance boost) or are using a slower interface, maybe you won't run into this. It sounds like it might not be an issue if you buy your cards from Silicom, either. But do be aware that at present there are configurations that can wind up hitting you with that kind of hidden cost, which may wind up favoring another vendor.
Hopefully, Silicom will change their minds on this.