I'll weigh in on this one, as I've done it before, where I also suggested the percentage.
Depending on the type of website, and how your client think it will generate revenue depends on what I would suggest! If he has a proper business plan that looks like it will produce money, then I would say go ahead. It's not any more work for you to keep it updated (I hope!), and if the money is coming in as expected, this can last for a long time, without you doing anything more.
If they are just hoping money comes in, I would not touch this type of deal. Ever again. Period. It can be damaging to your friendship if you are promised lots of money from a website, and you decide to cut a deal for the setup of it. I have worked diligently to prevent this particular scenario from happening, and would recommend you tread lightly.
For the latest one that I did this for, it was for selling some new-born puppies, purebred English Bulldogs. Apparently, they can be sold for anywhere from $2,500 to $3,500 each. I suggested 10% of the sale of each of the 4 pups, and started building the website. I thought I could potentially get $1,000 from a basic website with a photo album and contact form, as their plan was to sell them after 3 months. Unfortunately, within that 3 months, one died (mother dog rolled on it and suffocated it), and one other one was sold privately, not because of the website. The other two, they became attached too, and decided not to sell them. So, my net profit is $0, and my costs include the time I spent taking pictures, setting up the site, and buying/hosting the domain name.
As with any deal you make, make sure there is an exit strategy, and that you will get paid what you're worth. If you can make this website, deliverable, for only $100, then you are OK, everything else will be icing on the cake. THIS IS GOOD. If it will cost your more than $100 (whether it's your time, the hosting, the research, the physical or licensing components, etc), then this deal will probably not be a good one. Again, there needs to be a proper business plan that outlines how it's going to be making money, in order for it to make money.
What I would recommend, if you don't think that $100 will cut it, is specify a minimum payment per quarter. In this case, you may not need it, since it's only $100, but for other users, think about the minimums you need to keep something running.