Well because he's 21, he's getting better and in the last two seasons he's scored 32 in 66 games and had 10 assists in the Premier League.
It's worth mentioning that even when he doesn't score or assist, he's capable of absolutely dominating a centreback partnership by himself, and generally causes mayhem with his pace, size and strength
His main weaknesses imo are his passing (execution and timing) and a tendency to play without the required intensity. If he can consistently play with even the intensity of Sturridge or someone, not necessarily Suarez or Kuyt, he'd be much better than he is and easily worth £30m
Either way he's worth £30m to them. They know he works in their system, he's young, has a good goalscoring record and they're not going to attract anyone better for the price.
I think they'll end up selling him for more than £30m, depending on his contract at that time