I charge for total exclusive ownership based on what I estimate having the map in the portfolio would have earned me in future commissions by people who liked that specific map. Most people don't actually mean that they want to be the only one who ever shows the art but if people do request that from me, I offer it at a VERY steep price, higher than you are asking. Usually what people want is to know that I'm not reselling their world with their names on it, that it's watermarked, and they get the right to include it in their property, not a total ownership. I would charge around $500 for a commercial two page 3D building including hand lettered book map within two weeks, yeah... less if it weren't so fast.
For the record, I estimate around $35 an hour for my base prices, not $50+, but I know some people around here go higher. I found a decent goldilocks zone that works for me and clients I end up taking on, but certainly some of the best around here are likely charging double or more my prices. I fiddle around with rights increased payments mostly but try to keep the base hourly down for the sake of my recurring publisher clients and DM clients.
The basic map isn't too bad, it just depends how elaborate the decorations get, I guess. That could push it over the 10 hour mark for sure. I'd expect at least a few sessions on it and because of that I'd rather have the work spread over a month. I'd do it, I like the pitch, I think the end result of this one should come out very nice with your ideas, but the speed requirement makes me think you'll be better off with one of the keener newbies up there.
Most of us who've been doing them for several years end up with queues and time limits unless you come at a fated moment when they happen to have a slot available, heh.
Hope you find the right artist and fast!