That's a hard one. I did a lot of corporate shots for my work (we're pretty much one of the largest underground mining operations in the country) in my own time but I charged them a fortune because that's what they're used to. I charged them $4,500 for three days of shooting plus about 1,000 processed images delivered. I also charged them for servicing my camera and lenses afterward due to the dust underground (not so bad through the smelters and concentrators).

I'm not sure you'll be in that position with these guys though. It's a hard one for sure. Let us know how you go.