The midfielder moved to St James' Park in the January transfer window and has followed an impressive debut against Aston Villa with his matching-winning display versus Chelsea.
Pardew was full of praise for two-goal Sissoko after Saturday's dramatic 3-2 victory over the European champions and is in no doubt the former Toulouse man is money well spent.
"He has fantastic power but, with players, you never know about their attitude until you get them in and start working with them," said Pardew.
"But he came in like a giant and played like a giant. I remember watching him for Toulouse 14 months ago and thought, 'he is the one for us'.
"We have watched him for a long time but we decided to accelerate it because he was going to come in the summer.
"But, my word, he is already paying us back handsomely."