Rashford deserves a lot of praise for the growth he's shown during tumultuous times at the club and the chaos of managerial upheaval, but this thread is a reach and reads like the hope of fans, not the reality of the here and now.
To put a valuation to Rashford, you first have to look around the clubs that pay fortunes for talent, then you have to look at what they have and who they have spent on in the past and why. Then, you have to ask whether they would put equivalent money on the table for Rashford as usurper/supplanter, because no [true] fortune is spent on a substitute. So where does that leave us?
Neymar as potentially the best player on the planet sold twice for over £150m (yes, I'm factoring in all the underhanded payments).
Mbappe, as potentially the player of his generation, sold for over £170m already and any future sale, injuries and form notwithstanding, will be even more - invariably the new world record transfer.
Bale, after taking the league by storm, exploding in the CL and performing internationally.
Pogba, legitimately expected to be the midfielder of his generation and a future all-timer.
Cristiano Ronaldo, Juventus bought him to win the CL and raise their profile.
Coutinho, to Barca after a number of seasons of looking top brass.
Dembele, to Barca after a phenomenal season at Dortmund.
Hazard, after literal years of performance that some rate as the best in the PL.
None of this even factors commercial value, which obviously props up what's stated and catapults these players into a bracket clubs pay premiums for as a given.
Without the Champions League or a breakout international tournament or even absolute domestic-level dominance, it's absurd to place Rashford's monetary value over Pogba in our team. You first have to ask who would pay a premium for him as there only a handful of clubs who even entertain that notion: Real, PSG, Barca, Ourselves, Juve with the likes of City, Liverpool and perhaps even Chelsea not being clubs we'd sell to.
As of yet, Rashford does not have the profile for the Spanish clubs to come to the table with proper and correct offers; PSG are stable for the time being, besides which, they seek a more constant level of technical ability in their forwards... you're left with Juventus, far more likely to plunder Serie A than move on an unproven player from another league for an astronomical fee.
A player is worth what someone else is willing to pay. I would very much like to see a non-rose tint on where people are getting their Rashford valuations from and who they think would shell out Pogba money for him, and why.
As an addendum, I do think Rashford will have a genuine breakthrough season, at which point it would not be unreasonable or illogical to think his worth will take a leap and have him as a player the aforementioned teams would have on their radar as a starter, not an undercut substitute.