Cranky (https://crankyflier.com/2018/07/12/j...-a321-upgauge/) references JetBlue investor presentation that A223 has "25-30% lower direct costs per seat" and draws the conclusion that that flying an A223 would cost the same (in absolute amount terms) than flying an E90.
So: "If there are smaller markets where JetBlue thinks a 100-seat airplane is good today, then the -300 can fly those just as profitably as the Embraer can even with a bunch of empty seats. But if those markets can generate a little more traffic at lower fares, then that’ll all be gravy."