Clarke believes Foster could be worth up to a dozen Premier League points on his own.
"Ben's been a priority signing all summer," Clarke admitted to the Birmingham Mail.
"His performances last season were exceptional and it was important for the club to strike a permanent deal for him. It's a great signing for me and an even better one for the club.
"I think the supporters will be delighted we've been able to keep him on board and make him our player.
"In any league in the world you want a goalkeeper that is going to make match-winning saves, turn draws into wins and defeats into draws.
"A quality goalkeeper is worth ten or 12 points a season to you and that's certainly what we think we've got in Ben Foster."