The staff people should have been able to clearly explain that the print was 6 bucks because it was busted out of a calendar, and the others are 25 because they are on better paper, or whatever.
Other than that, they guy is completely out in left field somewhere if he thought he was buying something valuable for his 6 dollar "investment".
My wife has a great story of visiting the gallery in the 70's, being impressed by the photos on display, but picking out some postcards because the prints were way more than she could afford, and speaking to this interesting old gentlemen behind the counter;
Who turned out to be AA.