The boy is worth what the market will pay him. If he has a great World Cup, and I'm thinking he will, you can expect a few rumblings about clubs putting bids together for him. His price will rise, and the amount of salary he can command will increase.
As for his role in this World Cup. Personally I think Hodgson will start with him off the bench, looking to add his speed and energy later in games. Rooney was bloody awful last week and so was Welbeck, but I think Rooney gets the start. I think Hodgson will do the same with Oxlade Chamberlain. As a result I think Sterling will do great. 65 minutes in, in that draining climate, you send on Raheem and he runs past people like they aren't even there, fearless! He will do damage, just hope he has his "finishing" boots on.