If we were trying to sell. He's worth what someone is willing to pay.
If he was put up for sale tomorrow and Rashford wanted out, who's going to buy him, and for how much?
The problem when you're trying to sell these sorts of players for huge fees is there's actually very few clubs in Europe who have the sort of money to spend over £50m on a player. Pretty much just Madrid, Bayern, PSG and a few top premier league sides. Generally you don't want to sell to a premier league rival either. And if United deem him not good enough and want rid, why would the elite clubs in Europe want him when they're currently better teams than we are. And for nearly 100 million?
Occasionally you get lucky and a less wealthy club sell a player for a huge fee that summer and so it gives them some funds to bring in a replacement. But then you have the issue that these sorts of players think they're above certain clubs and expect to play for one of elite sides, even if those elite sides are not interested.
We may get lucky if PSG haven't noticed he's had 2 dreadful seasons in the last 3, but their interest in him looks to have cooled off, even with Mbappe leaving which opens up a spot. And no other rich club in Europe is buying him. And we'd be very very lucky to get the £80m we could have gotten over the last 2 or 3 years, he's no longer considered a young talent ready to reach the next level. He's soon to be 27, is very reliant on pace and lately he's having more dreadful seasons than he has good ones.