Per Sky Sports' Joe Shread, Sanchez has signed a deal worth £350,000 per week with United, and Mourinho said the huge contract was not the reason the forward decided to join the club instead of City:
"You have to put things in perspective—how much would Alexis Sanchez cost in a normal transfer with three years left on his contract? Look to the numbers now and I would say roughly between £100-150 million. Alexis could go everywhere, he had lots of choices, he decided to come here and you have to ask him why. Was expensive? Was cheap? Honestly I think it was fair. I think it's a very good deal for us."
Sanchez had five months remaining on his Arsenal contract before he left the Emirates Stadium for Old Trafford on Monday.
United paid no fee for the 29-year-old, as Henrikh Mkhitaryan joined the Gunners in a swap agreement.
It had previously looked as though the Chile international was destined to move to City and reunite with former Barcelona boss Pep Guardiola.
The Sky Blues unsuccessfully bid for him late in the summer transfer window but maintained their interest until pulling out of the running earlier in January, citing concerns over his wage demands, per Matt Hughes in The Times.
Mourinho, who penned a new contract with United on Thursday, offered a unique view on City's failure to sign Sanchez, per Shread:
"Alexis reminds me a little bit the metaphor when you see a tree with amazing oranges in the top of the tree and then you cannot get there and you say I got the lower ones because I don't like the ones in the top. You like the ones in the top, they are so nice and so orange and so round, so full of juice, but you cannot get there. So you say I didn't want to go there or I prefer the other ones. I think it reminds me a little bit of that story."
Sanchez could make his United debut on Friday, when the Premier League giants travel to Huish Park to take on Yeovil Town in the FA Cup fourth round.
It will be interesting to see where Mourinho would play Sanchez against the League Two outfit.
His preferred position during a successful three-and-a-half-season spell at Arsenal was on the left flank, but Anthony Martial has been thriving recently in the same role at United.
Sanchez is versatile enough to play in a number of positions and still be effective, and Friday night's clash will provide the first indication of how Mourinho plans to get the best out of him.