The Blue Book of Gun Values can serve as a starting point, but you may want to ask other gun dealers in your area, if you know of any that you can trust for a fair guess as to value. A rifle's value can be very different, depending on the local collectors' market, and favored methods of hunting in your area. If it's still on the dealer's shelf in a couple of weeks, it isn't worth as much as he'd hoped, because nobody is paying that much for it. Then again, it might be gone, like the 375 H&H I should have snapped up a few months ago. If you really want that rifle, and you have the means, haggle a little, but go ahead and get it. You'll forget the price in a couple of months, and the 375 Winchester is a good lever gun round. It'll handle a lot more than just deer at close quarters, with the right bullet and load.
In my local gunshop, the owner told me that he can't get much more for a .375 Winchester than he can for a .30-30, but what you described may be a collectable rifle, or more popular in your neck of the woods.
