Even though they are not GNs but BNs, $4 is still a decent price. Places around here charge anywhere between $3 and $10 for common BNs. Besides, it was an admirable thing to rescue them.
I once considered rescuing a nice group (probably about 8) synos from Wal-Mart, but a lack of quarantine and space in my populated tanks made me not do it. I think they were $4-5 each. Even for common synos, not bad IMO.
While I definitely agree that Wal-Mart should not be permitted to carry fish, I still shop there. Poor grad students have no other viable options.