Could the U.S. purchase the northern Mexico states?

[Virginiá]

If anything, we should be giving it back what we stole. Brought us nothing but trouble.

That doesn't seem fair. If you include all the territory taken from Mexico in one fashion or another, you get this map (Green for partial MX, red for all MX, not sure about New Mexico entirely):

Obviously Texas has been vital to our energy concerns for some time. California is also extremely important to our economy/country as a whole in many ways and has been seen as a bright spot for the country. No comment on others (Though CO is beautiful!)

[Virginiá]

But would it be unconstitutional for the individual states to have an independence or annexation vote, and then for Mexico to be financially compensated by the US afterwards?

I'm willing to bet that if Mexico legitimately wanted to do this and it was unconstitutional, they would just get an amendment passed to make it constitutional. It's not likely a situation would arise without a significant amount of support.

But for the record, the idea itself is ridiculous.