Advice on being approached about design?

[viral-divinity.livejournal.com]

Hello all, I find myself in need of some advice.

Earlier today, I was noted by someone on FurAffinity, who said they had stumbled on my page and inquired about a commission. But it wouldn't be anything that I'm used to - they requested conceptual art of a computer case design they were going to be building. And they would like it to be a collaborative project. (Which I have no objections to! None at all; it's just different.)

They already had something in mind, and my designs vs their designs would be more of a back and forth collaboration in setting up the design groundwork for their computer case. They'd be able to work out the dimensions and whatnot, as they would be building the entire computer from scratch, and I'd be in charge of getting a mock up concept of what the case wold look like and they'd go from there as additional ideas/concepts were thought up.

My question is, how much should I charge for this type of work? I'm only used to doing personal illustrations for my clients, and If applicable, here is what I normally charge and do for my clients, in my ToS journal on furaffinity, prices and examples included.

I suppose I'm just a little lost on where I should go with this - how much should I charge, mainly. Or if I have anything I should be wary about in this type of commissioned conceptual art, as I've really never done anything like this before.

Thanks so much for your time in responding - if any additional information is needed, I'll be sure to either edit the main post or reply in the comments. :)