It’s not a 1st edition in the sense that it has 1st edition cards inside. Rather it’s a variant of the black triangle packs which had used the black triangles to cover a 1st edition stamp. This variant is without the black triangle. Inside is contained unlimited print cards only… The reason it went so high is that they’re quite rare and very tough to find variant.
Feel free to read about it here: 
www.elitefourum.com/t/identifying-shadowless-unlimited-base-set-boxes-packs/16993/1