Since I've now used (or test mowed with) 6 different ZTR mowers on my property this spring, there is a huge difference on how they mow on hills. However, for the most part, each ZTR I tested seemed to be exceptionally well built. I even tested 2 different eXmark units, one gas and one diesel. If mowing is the only chore, a ZTR will mow more than 2 lawn tractors. But, as you mentioned, the price difference is steep.
There seems to be just as big of a jump in cost and productivity when going to a front mount ZTR. I spent a considerable amount of time and money this spring learning this fact. There is a big jump from a lawn tractor to a ZTR, and there is a big jump from a mid mount ZTR to a front mount ZTR.
However, each step depends on your needs. If you need to tow something and need to use a front blade to push snow etc., a lawn tractor would work better for you than a ZTR. If you have a smooth lawn, no lake, no steep hills, and not many inside corners, then a mid mount ZTR will be all you will ever need. If you have a lake, lots of obsticles, a lawn that isn't exactly smooth, inside corners to mow, trees and bushes to mow under, and some significant hills, then a front mount ZTR is needed. I can literally stop, turn in complete circles, and continue mowing with my ZTR on the side of some hills that I wouldn't dare attempt to cross on my tractor. It all depends on your particular application.
I have not been on what I would consider a poorly built ZTR, mid or front mount. I didn't want to spend the extra money, but with my layout, a mid mount ZTR just wouldn't work. For my 2nd ZTR, I had to go with another front mount. Like I mentioned, from experience, you have several steps. First the lawn tractor, then the mid mount ZTR, then the front mount ZTR. There is just as much of a jump in abilities and production (and cost) with each. You just need to figure out which will fit your needs. There is no sense spending money for something you don't need, but there is also a painful feeling to spend quite a bit just to find that it won't do what you need. I can't say it enough; demo what you are considering on your own property. Then, make sure you use the demo model in all areas you intend to mow. After that, then you will be ready to buy what you need.