Advice for a logo commission?

[ukthewhitewolf.livejournal.com]

Hey, I've got someone who's contacted me about a commission for a logo for a friend of their dad who runs a Dutch radio station. With the language barrier the person who contacted me is the one negotiating the commission and obviously I'd rather sell the rights to the logo image so they can use it for whatever they want and I don't need to worry about not knowing whether they're using it commercially in a way that hadn't been agreed upon prior and basically avoiding what could be a massive headache all around.

My problem being, I've never sold the rights to an image before as I always retain my rights to the images I do for regular commissions so I've not got a single clue what I should charge for the full rights to the image?
I'd charge about £30-£40 for the logo itself, but no idea what I should charge for the rights. I tried looking about on my own but it was pretty confusing and figured just asking for advice would be easier.