The most valuable comic I own is Action Comics #242, first appearance of Brainiac. I need to get it 'professionally' graded I guess... I would say it's worth around $1000 to $1200.
That's the cover of it (courtesy wikipedia).
After that I have no valuable comics at all lmao... next one after that is first apperance of Gambit in Uncanny X-Men #266. What a big jump in value eh?