The 2 Promo Codes that I found and actually work are for 5% off. They are 5FEB8 and cmz1541. I think the reason your 9:40pm flight out of MKE - OMA is more expensive is because it is almost sold out for the 26th of May. Sorry I couldn't find a larger discount. Maybe someone else on this forum can find better deals. Also Midwest Customer Service might have a larger discount that they offer to students. You may want to give them a call.