It's true that Nicolas Jackson valuation is overstated, and I disagree even his performance is mediocre. Without an ideal ball supply, Jackson often misses opportunities. However it's no wonder Chelsea set such a price, as almost all of their players have been positively impacted by their victory in the CWC Final. Man United is not a club with maximum financial support, in fact currently Man United only recruits unknown players.
For a player of Nicolas Jackson's caliber, Chelsea's asking price is clearly very high, even though they have benefited from the Club World Cup. Manchester United has set a modest budget in the current transfer window, having spent heavily on players last season, but the results were disastrous. This is evident in Manchester United's current transfer process, which demonstrates their careful negotiation, as evidenced by the two players they secured.