He has been tremendously consistent for Porto and is by far our most important player. He won't be released for cheap, though we must manage his expectations (he has been a tremendous professional in the past 4 years, extremely polite to the club, but it's natural that he, at 25 years old, wants to prove himself in a bigger league). I think he'll be awarded that chance, but fortunately he doesn't seem the type to fret if the club decides not to sell him in case no decent offers arrive.
A lot of unrealistic muppetry on the Porto forums, where many regard him has the 3rd best player in the world and expect he will be sold by something like 60-70 million euros. I really don't buy into that.
As good as he his for us, and despite the easiness with which he destroys teams in our league, he isn't even a guaranteed started for his national team. That is sure to have some weight in his market value. Hence why I think it's laughable that some people really think he's worth 70 million euros.
The most expensive player to ever leave the Portuguese league was Falcao at €40m (which by the way Atletico is having tremendous difficulty to pay and might be therefore forced to sell him). Though completely different players I think Hulk should be valued similarly and would consider a good deal if he was sold for that kind of money. I can imagine mass rage ensuing among Porto fans if that happens though, hence why I'm not 100% sure he will be sold.
The only way I see him leaving for more than that is if a club from a lesser league goes really over the board, like PSG or Anzhi. Hulk really wants to go to a better league though, and I think a LOT of money would be needed to convince him to go to such clubs.