How much should I charge a colleague for a translation project?
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.