I would just be straight up with them, say you've never been paid for your work before and ask them what they think a reasonable offer would be. If the offer is insulting, then decline. Like you said, this could lead to a collaboration in the future. As you said already, this is a hobby for you, so anything you make is still a profit from doing what you love. You don't need to break the first time, but don't settle for anything less than you feel you're worth. I'm no professional photographer, but I'm sure just like with anything there's a learning curve. This could be the beginning of a new journey for you.