That is according to former Newcastle goalkeeper and ESPN pundit Shaka Hislop, who values the midfielder at just half that price.
Manchester United are keen to secure the service of Longstaff, who has only ever featured in nine Premier League games for the Magpies.
The Red Devils are expected to want to pay a maximum £25m for the 21-year-old with boss Ole Gunnar Solskjaer vowing to walk away from over-priced targets.
Old Trafford’s hierarchy are also refusing to pay Leicester’s £90m valuation for Harry Maguire – but Longstaff’s price is the clearest example of what they believe to be unrealistic fees for domestic talent.
United made a formal approach for Longstaff only to be left apparently shocked after being informed that he would cost the same amount as they spent on Aaron Wan-Bissaka to prise him away from St James’ Park.
Solskjaer remained adamant that the club will not be held to ransom.
“We’re still in the market looking to do some business, but we’re patient,” he said.
“We’re Manchester United. We don’t have to sell and don’t have to overpay.
“You have to have the right players at the right price.”
“I don’t know why the price for Sean Longstaff is set at the £50m mark”
And Hislop agrees with United’s stance on Longstaff, insisting that there is no justification for Newcastle’s asking price when the player has such little first team experience.
“I don’t know why the price for Sean Longstaff is set at the £50m mark,” Hislop told ESPN FC.
“Apparently it is because of the Wan-Bissaka deal who is of a similar age and that is where it looks like Newcastle are setting their sights.
“The difference for me though is the fact that Wan-Bissaka played 35 games for Crystal Palace last season and we know exactly how good he is.
“Longstaff has played only nine games for Newcastle though and so it just does not make any sense.
“But if you are Newcastle I am not so sure that you are that motivated a seller, they are sorting out a lot of issues at the moment.
“If I am Manchester United and I am trying to stock up with young, English talent who can do well in the long term, I don’t think that I would go much higher than £25m.”
United are still expected to press ahead in their pursuit of Longstaff but have made it clear that they will not pay Newcastle’s £50m asking price.