TV Shows on DVD

Why is it that I can get a season of Hercules for prices varying from $39.99 to $69.99 and yet a season of Star Trek: DS9 can vary from $89.99 to $149.99? These prices just don't make any sense! And all the Buffy seasons were on sale for $29.99! I was seriously considering buying a bunch of them, but what I really wanted was some DS9, because I had a big DS9 craving. So I didn't end up buying anything again. Oh well, if I did, I would just spend my whole week watching TV again. I'll just spend my money on what I usually end up spending it on: Cat Food!