The Terps fell behind in the first inning for the fourth straight game.
Right-hander Sean Heine was tagged for five runs in the first inning.
It's the third time Costes has earned the award this season.
Maxwell Costes and Justin Vought each clubbed three RBIs.
Coach Rob Vaughn was ejected for protesting the Wildcats' go-ahead run in the eighth.
The Terps issued 12 walks.
Justin Vought launched a grand slam as part of a seven-run fifth inning.
Maxwell Costes and Randy Bednar belted 3-run shots to win Saturday's nightcap.
With the help of sloppy Illini defense, the Terps posted four ninth-inning runs.