I don't think Wilshere has as much ceiling as we make out to be. I think the form he had at the start of the season was good but he was only our best midfielder because everyone else was out of form. It's a tradition with English players really, that spark very fast but you don't really see them develop that well later into there careers bar a few exceptions. I don't think Jack will become a complete midfielder or ever reach world class but he has the potential to be one of our best players no doubt. Most likely hood is that due to his reoccurring injury problems he won't reach his potential. He's hardly played any games for us this season so yes, I would take 25m for Wilshere but in a window where Lallana goes for around 28m I'd expect Wilshere to hit 30m too.
There are better ready made players out there now, and certainly for sure better youth prospects too.