Normally, you wouldn't be getting royalties for something like this. Instead what you'll be selling is the art plus all rights to it's future use. While some here might suggest really jacking up the price, two to three times what you'd charge for a regular commission of the same complexity is probably fair. You will need to draw up a simple document signing off on the republication rights and such. There are no doubt sample forms to fill in on the net if you Google for them. They'll need that should they wish to trademark the logo.