Pardon me for being cynical, but are they proposing to pay you some money or merely to give you and your work some free publicity?
If they're making a serious business proposition there should be a reasonable amount of cash involved. As for the publicity, that's great, but you can't take it to the bank. You can't even put a value on it. What you can do is take good photographs of your work, including some after installation and put them in a portfolio to show to clients.
I'd reply with some drawings and a rough estimate for the job. It's just a principle I won't let go of. I'll build gifts for friends or maybe something to donate to a charity auction, but if somone wants to book my skills and my time - there're going to have to pay for them.
Being a curmudgeon comes naturally for me :soapbox:
All the best,
**Now holding auditions for a catchy new signature**