I'm trying media edit 3 (60) on trial. Express is, unfortunately, too expensive. Are there any that are under 50? I'm probably fighting a losing battle, but it's worth a try.
You could try Blender, although it's video editing is kinda simplistic (it's more of a 3D modeling tool). I think you're best off just staying with iMovie. I just checked MacPorts for Cinelerra, sadly, nothing. I've never used it, but it looks reasonably decent.