Pep Guardiola's side have agreed a move for the England international that will see the Leeds United star leave his boyhood club for a fee of £42million, plus £3million in additional clauses.
The 26-year-old has been a central figure at Elland Road since breaking into the first team fold under Marcelo Bielsa, and has been heavily linked with big-money moves to a number of clubs in the Premier League.
According to Bent, the deal agreed with Leeds is excellent business for City who have filled a crucial gap in Pep Guardiola's side.
Speaking on talkSPORT Drive, Bent said: "For Kalvin Phillips it's fantastic being a part of arguably the best team around in Manchester City. He's a top player.
"I get it, they've always had two holding midfielders. Rodri's emergence into a top, top holding midfield player and Fernandinho leaving means that they need someone else there.
"For Kalvin Phillips, and his international aspirations given how key he was for England's success in getting to the final of the Euros, if he starts playing for this Man City team he could become a mainstay in the England team.
"£42million for Kalvin Phillips? It's bonkers.
"When I heard this rumour a couple of months back, I was hearing fees like £75million, £80million and fees like that, but if they can get a player of that quality for £42million, wow.
He added: "I can't believe the fee.
"In today's market where people are splashing the cash left, right and centre, £50million here and £70million there, £42million for an England international."
Asked if that pushes Declan Rice's value down, Bent said: "If you ask me would I rather sign Rice for £80million or Phillips for £42million, I'm taking Phillips.
"£85million is a large chunk of money for any team. Declan Rice is a better player, but for Manchester City a lot of people are saying that he's going to be a squad player.
"When you go to a club like Man City, unless you're Kevin De Bruyne, you're going to be a squad player.
"That's a worry for the Premier League, that is. Phillips and Haaland.
"This has come from nowhere which makes me think that they've been working on this for a while."