If two things are comparably expensive, they cost about the same — you can set them side by side and the degree matches. Comparably means “in a similar way” or “to a similar degree.” It is a bit formal; in speech people often say “about as… as” or “similarly.” Use it when the likeness is in how much or how far. Though actually it is not the same as comparatively: comparatively cheap means cheap when you set it next to something else (maybe still not cheap in the abstract); comparably cheap would mean cheap to a similar degree as the other thing. That tidy mix-up is everywhere, even in careful writing.