How much should I charge a colleague for a translation project? I have come across a situation that I'm not entirely sure how to handle and am curious if there are any norms in the United States for it.

I teach in my university's foreign language department.  A colleague in another department approached me to see if I could assist in translating an article for a book that he was working on.  The article is in the language that I specialize in and also very closely related to my research area.

If it were just a paragraph, I would of course do it without question.  However, given its length (about 13k words), I'm not sure how to approach.  He has offered to pay, but is it normal to charge the same amount as I would were it a private business?  Or is it expected to offer a reduced rate in the name of "service"?
I imagine the answer here is slightly subjective, but my personal thoughts are that you should charge whatever you feel your time is worth. The industry price is probably a good starting point -- don't undervalue your time for sure. And remember the time and energy that went into developing your language skills too, not just the time it will take you to translate the article.

That being said, the industry price is just that -- a starting point. If you want to reduce the price for a colleague, you can certainly do so. You may have political reasons for doing so (you want their support on something within the university), or they may have services to offer you in return (as in a barter economy). I'd caution you against reducing your price too much -- you don't want to be inundated with requests like this in the future.