The Chilean striker would earn between 400,000 and 500,000 pounds ($555,000 and $690,000) a week before tax by moving to Old Trafford, according to The Sun, the Daily Telegraph and other newspapers.
The 29-year-old's wage demands are said to have scared off Manchester City and Chelsea, leaving United as his main suitors.
The Daily Telegraph reported that United have offered Sanchez a four-and-a-half-year contract and he would move for a £30 million transfer fee -- it is relatively low because he could leave for free at the end of the summer.
The deal for the former Barcelona player would also likely involve a £20 million signing-on fee and a £10 million agent's fee.
The Sun reports however that Arsenal could still refuse to allow Sanchez to leave if they do not find a replacement.
However, Arsenal manager Arsene Wenger at his Thursday press conference said the deal was probably going to be done.
"I worked on transfers for 30 years so it is likely to happen but at any moment or minute things can break down," said Wenger.
"As long as it's not over the line you have to accept it may not happen. These kind of things are not guaranteed."
Wenger, whose failure to keep key players and perceived inability to replace them with similarly talented stars has seen support drain away amidst sections of the club's supporters, said he had no problem with the amounts being mentioned in the press of what United were prepared to pay two-time Copa America winner Sanchez.
"I respect United because they generate the money they pay by their own resources," said Wenger, who revealed German star Mesut Ozil would not be joining Sanchez in quitting North London.
"United is a club very well managed financially and of course on the pitch so that's why I don’t have any problem with the money they pay."
United's unsettled Armenian midfielder Henrykh Mkhitaryan -- subject to several public blasts from United manager Jose Mourinho this season -- has been suggested as being part of the deal.
Wenger did nothing to downplay that declaring his respect for the former Borussia Dortmund star.
"It's a possibility it's because I like the player," said the 68-year-old Frenchman.
"We played many times against him when he was at Dortmund. He appreciated the quality of our games. The wages would not be a problem."
United are second in the Premier League, 12 points behind runaway leaders Manchester City, and Sanchez's capture would be seen as a statement of intent from a team already paying huge wages to French international midfielder Paul Pogba and Belgian striker Romelu Lukaku.
Arsenal, struggling to compete for the Champions League places and knocked out of the FA Cup, have been linked with Borussia Dortmund's wantaway forward Pierre-Emerick Aubameyang and Bordeaux's 20-year-old Brazilian striker Malcom.
Wenger, though, ruled out a move for Malcom but hinted Mkhitaryan's former team-mate Aubameyang could be a valuable addition to a side which has seen their record signing Alexandre Lacazette fail to score the goals he was expressly bought for -- he has just eight all season and hasn't scored in his last nine appearances dating back to December 2.
"This kind of thing is better when it’s secret and when you don't comment before announcing it when it's over the line," said Wenger.
"Would he (Aubameyang) fit in?.
"Yes because character can be a very positive note . Overall you look at the achievement of a player over his career and when it's been positive the character has been used in a positive way."