You're one of the 52 people that have the Erikson lettered edition from Subterranean Press? I'm so extremely jealous. I was able to get in at MT, so I should have numbered editions of MT through CG once its all done. But the lettered editions, those are truly beautiful books. Can you post pictures of your collection somewhere?
Going to have to disappoint you on cost. $350 is probably a low end estimate for something like this. A comfortable number is probably double that. I will probably have spent $700-$800 once its done. The biggest issue is economies of scale. The materials alone are extremely expensive to buy for just one book, so unless you're making a 500 book print run, done on machines rather than by hand, it becomes egregiously expensive. I got to learn this lesson first hand.
Mind you, almost half of that cost was the super premium leather I purchased. Cheaper stuff can be used, and not to bash on your books, but the leather that Subterranean Press/Easton Press/etc. isn't nearly the same 'quality'. You can still make very high quality books with lesser leather, but you could still end up with $100 - $200 worth of leather to purchase for each book.
Doing this kind of thing is very time intensive and takes a lot of skill to end up with something that doesn't look like garbage - skill I definitely don't have. I'm thinking the bare minimum price would be closer to $500 per book, but I haven't discussed this with my binder. If you're wondering how I came to such outrageous prices, I have another thread documenting much of my journey here:
http://www.second-apocalypse.com/index.php?topic=1371.0.
I was getting quotes from $700-$1000+ from bookbinders all around the country - and some flat out refused. I thought I could do it more cheaply going the route I did, and in the end it really didn't save me any money. It was fun, actually getting to craft it myself with guidance, but still expensive. Most of the cost is in materials, so its really hard to reduce the price.
But hey, I'll see what needs to be done. If there is a $350 option, I'll look for it. I recently found someone online that does custom books for cheaper, but they use some non-conventional processes that I'm not sure would have the same effect.
Make sure you set up an email address for your account here, or link the PMs to your email, so that I can reach out to you with more information as it comes.
A clam-shell box (aka traycase) would be an additional expense. I'm tapped out as far as budget goes for this project, so I never looked into the cost of that, but I can do that for you as well, just to see. They really are the best way to keep your book in pristine condition, but I think its kind of a shame to hide it after putting all this work into it.