Barkley, who was close to signing for Chelsea last summer, made the switch from Merseyside to West London this month in a deal that Cahill has described as "phenomenal".
England international Barkley failed to have an impact with the Toffees in the first half of the season, but Cahill has said that he is confident his new teammate can flourish with the Blues.
"Phenomenal. It's a great signing - a bargain," Cahill told the Evening Standard when asked about the midfielder's move. "When you look at the market in this day and age, it's a snip for a player like him.
"I've played with him, and trained with him, and I know what he's capable of. And so does everyone else. Does he have a good left foot and right foot? Is he strong, is he quick? Can he finish? Is he comfortable on the ball? He ticks all the boxes.
"Now it's just ready for a platform for him to go out and express himself. This club will give him that, like they gave me and others. It's down to him to take it. But it's a very exciting signing and for the money, it has to be a bargain."
Chelsea face Arsenal in the EFL Cup semi-finals on Wednesday, before Antonio Conte's men take on Leicester City in the Premier League on Saturday.