Well, that price does seem a bit too high for Nicolas Jackson. It's not that I doubt his abilities. However, his position at Chelsea is actually a bit uncertain. Chelsea has already signed several new players for the position usually occupied by Nicolas Jackson, so Chelsea should be able to lower the price a bit if they really intend to let him go. Or maybe Chelsea doesn't intend to let him go. I don't know.
Such a high market value would make it difficult for any team to make an approach unless his name is highly discussed as a potential player after hitting his peak form. Now, considering the various clubs interested in Nicolas Jackson, none are willing to pay that price, so Chelsea are forced to keep him, whether they like it or not. Two representatives in Italy did express interest in him, although he turned them down, but no negotiations progressed, and I believe that's because his value is so high. Manchester United won't spend any more money on signings because they're already quite cautious in approaching the players they want.