The Netherlands international will become the most expensive defender in football history when he makes the move from St Mary's to Anfield on January 1.
The deal brings an end to the heavy speculation surrounding the 26-year-old, who had been heavily linked to the Reds and Manchester City among other clubs since the previous transfer window.
Southampton have been convinced to part ways with the centre-back, however, and, having been asked his opinion on the deal, Carragher believes the Reds are getting good value when compared to United's £40m signing of the much-criticised Lindelof.
He Tweeted: "Not sure, but double the price of Lindelof sounds cheap to me!!"
Neville, meanwhile, was initially shocked over the fee the Reds are paying for Van Dijk but believes it could well be a shrewd bit of business if the centre-back helps solve the club's defensive issues.
"It's a lot of money! When I first heard it I thought 'wow, £75m'. I'm not surprised by the actual transfer but that is an incredible amount of money," he told Sky Sports .
"Liverpool need defenders and have to pay to get him. That's the way the market is and it isn't going to change.
"It sounds like a fantastic deal for Southampton. But if Liverpool can stop their creaking in big matches when put under pressure then it will be a good deal for Liverpool as well because people will forget about the money that has been spent in a year or two if they can win trophies and get the defence right.
"Liverpool have to get the defence right, in the bigger matches they do stress quite quickly and can crack, so from their point of view, paying that sort of money hopefully it pays off, but for Southampton they would snap your hand off for £75m.
"Over the last few years they have lost some good players for good money but nothing at this level. It seems like a fantastic sell.
"I was surprised in the summer when I saw £50m for Kyle Walker but he has done brilliantly and it's just the way the market has shifted to an incredible level that shocked me."