Jurgen Klopp worked to cure the Reds' defensive issues by signing the imposing Van Dijk for a world record fee for a centre-back.
The ex-Celtic star was heavily linked with a move to the Etihad Stadium but eventually moved to Merseyside 12 months ago, helping the Reds reach the Champions League final in his opening six months at the club.
The Amazon Prime documentary 'All or Nothing' which followed City throughout the 2017-18 season showed footage of chairman Khaldoon Al-Mubarak questioning Southampton's asking price with director of football Txiki Begiristain and chief executive Ferran Soriano.
But Guardiola said ahead of the visit of Liverpool on Thursday: “He's an exceptional player.
“When a player costs whatever it costs and it works, it is well worth it.
“When you pay less and it doesn't work, it is so expensive.
"If he can play every three days with no injuries and play consistently, it is well worth it.”
Already unbeaten in their opening 20 Premier League matches and seven points ahead of defending champions City ahead of the summit meeting, Liverpool can put one hand on the title if they beat their rivals.
Having lost three of their last five league games, Guardiola says City cannot afford to slip-up again and allow the Anfield club to increase their lead at the top.
"We know how important it (the match) is,” he added.
“If we are not able to win, especially if we lose – 10 points, goal difference 11 – if they continue that run it will be so difficult.
“It's almost a final for us. For them, nothing changes."