From on- + cost, or from Middle English uncost (“additional expense”), from Middle Dutch oncost (“expenses, overhead”), equivalent to un- + cost. Cognate with German Unkost (“expenses”).
"oncost." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/oncost>.
oncost. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/oncost