Sorry, no easy answers here.
i'm basically doing this translation job as a favor to an intermidiary.
If you're doing it as a favour, do it for free.
If you don't want to do it for free (which I would entirely understand), you need to come up with a price, based on the estimated amount of time it will take you to get the job done.
i'm doing this in my spare time.
In whatever time - it's your time.
basically, i'd just like to know how much people usually charge for a marketing oriented translation of a commercial web-site and promotional material.
How much is a BMW? Can't tell? Exactly.
It's impossible to give you even a rough estimate without having analysed the texts.
You may want to try the ProZ.com rate calculator - admittedly, it's simplistic, but could well be useful to get a preliminary idea.