According to this DBEDT document, the Kaua`i rates are similar to HECO subsidiary rates for the similarly sized markets on Hawai`i, Lana`i and Moloka`i: http://energy.hawaii.gov/wp-content/uploads/2011/10/FF_June2013_R2.pdf

I don't see an inherent reason a cooperative couldn't also realize economies of scale in a large market like O`ahu.