I the the problem is that with any comic book movie you have a limited budget.
Comics are sort of movies, where the writer and artists can do whatever they want because there is no budget on special effects, or actors, or settings,
but when you make a comic book movie you take a story that had no limitations on budget for the most part and add in whatever budget the studio feels comfortable giving you.
With FF2 I don't think they had much faith in it so they gave it less money than it required.
First you need money for the special effects for the FF's powers, then you need money for the Silver Surfer who was all CG, then you need money for the fight scenes, then you need money for everything else including Dr. Doom, the cast, and so on.
Galactus would have probably cost a lot to make too.