Thistle Seed has always been expensive, and there have always been the type that is heated or coated with some kind of premerge to keep it from sprouting all over your gardens.
(along with the non treated seed that will germinate all over)
Going to a local hardware store of any type is probably the most expensive place to buy it.
Going to Lowes, HD, Kmart or Walmart is a better deal.
Our local Agway has a 'Feathered Friends' program that greatly reduces the cost of feeding birds, including Thistle Seed.
Moral of the story: Shop Around.