Depends on what you want to do. Am I reading it right that you are looking for a hard boot vs a soft boot? I use softies as I like the surfy feel and flexibility you get with them. They are also cheaper than your hard boot alternatives.
For stiffer soft boots, the Salomon Malamute, Vans Cirro, and some K2 models (forget the names) are good boots. I highly recommend the BOA Focus system for splitting in softies. You can loosen the uppers by popping the dial for skinning and tighten them in a flash when you are ready to drop in. Much easier than even the speed lace systems.
For hard boots you are going to be hard pressed to spend under $600. That is just the way it is. Fortunately, you don't need the high end down hill performers. You are looking for the softest flexing models. The Scarpa F1, Dynafit ZZero, something like that. Basically a three buckle boot vs a four buckle (stiffer). Backcountry.com has a good selection.
Since I am not a hard boot guy on the split you might want to get some advice from actual hard booters on spltiboards. Check out the forums at Splitboard.com. You should be able to get plenty of advice over there that is geared more towards what you are looking for than here. I might be the only splitter on this board at the moment. Lots of gear heads at splitboard.com and pretty much everyone is a splitboarder.
People still use hard boots? I thought those died with boarder cross racing?
Anyway it is true that it is hard to find a stiffer boot that holds up. I prefer a medium flexing boot and end up replacing mine every 1 to 1.5 seasons. Gets expensive at $300ish each time.
A little trick I've learned is to cut pieces of plastic and slip them into your boot (between the shell and the liner). I use 3 pieces in each boot, two pieces are L shaped and go on the sides of the ankle up the leg. The other piece goes in front under the tongue. Helps a tone!! If you just want to stiffen up the forward flex then all you need is the tongue insert. If you just want to stiffen up the sideways flex then only the two side pieces. You get the picture.