Really depends on where you're buying from. From what I see at least, some marines like Vasquez are expensive beyond belief. I mostly see the humans being more expensive than the aliens. Though that's because I usually hunt down good deals for multiple aliens. Some older rarer human figures can be more expensive than old aliens. That's what I've seen, could be a different situation depending on the location.
Though with newer releases it's about the same between humans and aliens.
P. S. The Neca Newborn is weird. Depending on the angle, it looks better and worse than the one in the movie, at the same time! I'm glad I'm not getting that one...