Wow! A bunch of spinach is only ninety-eight yen!
The kanji refers to a bundle, but this refers to a counter for bundles. Put this on a number, and you have "1 bundle" or "2 bundles" or even three, oh my!
Saying "1 bundle" and "2 bundle" takes too long, so you decide to call these bundles what they look like: わ for walrus (わ)!