A.T. Robertson Commentary Mark 14:11

"And they, when they heard it, were glad, and promised to give him money. And he sought how he might conveniently deliver him [unto them]." — Mark 14:11 (ASV)

And they, when they heard it, were glad (ο δε ακουσαντες εχαρησαν). No doubt the rabbis looked on the treachery of Judas as a veritable dispensation of Providence amply justifying their plots against Jesus.

Conveniently (ευκαιρως). This was the whole point of the offer of Judas. He claimed that he knew enough of the habits of Jesus to enable them to catch him "in the absence of the multitude" (Luke 22:6) without waiting for the passover to be over, when the crowds would leave. For discussion of the motives of Judas, see on Mt 26:15. Mark merely notes the promise of "money" while Matthew mentions "thirty pieces of silver" (Zechariah 11:12), the price of a slave.