The sliders are steam-grilled and flash-frozen. You can choose from three varieties: Hamburgers, American Cheeseburgers, or Jalapeno Cheeseburgers.
Prices range from $127.95 to $170.95 and include frozen gel pack packaging and overnight shipping to the continental U.S. The White Castle website has some pretty detailed instructions on how to steam heat the frozen sliders.
I imagine these would be a little fresher than the frozen ones from the supermarket. A lot of the cost seems to stem from shipping and packaging considering 100 sliders from a White Castle restaurant proper costs $69 (varying with location).