Costello was signed to a one-year contract, alongside Oilers linemate Jack Combs, by the Colorado Eagles of the ECHL on August 18, 2011, for the 2011–12 season.
[4] Despite his injury, Costello finished fourth in league scoring and was selected to the First All-Star Team and chosen as the ECHL's Most Valuable Player.
With the conclusion of the NHL lockout inducing significant roster movement, Costello was signed again alongside Combs, to an AHL deal with the Sound Tigers for the remainder of the season on January 9, 2013.
[13] On May 21, 2017, he signed a try-out contract with the German team Iserlohn Roosters from the Deutsche Eishockey Liga (DEL), rejoining friend and former linemate, Jack Combs.
On March 23, 2018, with the conclusion of his contract with Iserlohn, Costello signed as a free agent continuing in the DEL on a one-year deal with Krefeld Pinguine.
[16] Following two seasons with the Pinguine, Costello continued his European career by playing on a shortened contract with Norwegian club, Stavanger Oilers of the Eliteserien.