Thank you all very much for your responses. I'm looking into all the potential artists you've mentioned.
The reason why I asked is because we've approached Gary Tonge (
http://www.visionafar.com/) who is incredible. He said he's absolutely open to doing some commissioned artwork and wanted to know what kind of budget we were working with. He's a professional artist who has a day job, so this would be something he'd do in his free time. I don't want to "slap him in the face" so to speak by low balling him, but of course we'd like it to be as cheap as possible haha. It's a tought thing to gauge but I asked him how much time he would spend on it. If it takes him a day to whip something up, then maybe his time is only worth a couple hundered bucks, but if he's going to take a whole week to deliver the quality of work he has on his vision afar sight I suspect it will be much more. Would it be reasonable to ask him what his time is worth or is it up to us to determine the budget and see how he responds? Naturally, if it's beyond what we think we can afford we'll start looking into some more affordable options.