There are a lot of great 6-7" blades that can be had for the $100 - $150 range. Check out some of the custom makers, like Koyote, Walter Davis, Stomper etc.
I'm not trying to offend the OP here, but referring to the need for a zombie killer, kind of sounds like not really knowing what he wants to do with the knife, just wants something really thick and tough. In my experience, whenever I bought a knife that I didn't really know what I'd do with it, it just ended up collecting dust. A $300 dust collector is a sad thing IMO.
Really think about what you want to do with the knife on hand. This is even more important with a 6-7" blade, the so called mid-range, which can be great "all rounders" if designed right or "not useful at anything" if applied for the wrong purposes.
For productions, RC-6 is a good blade with the right thickness for its length. Ranger - RD6 will give you a 0.25" beast as will a Scrapyard SOD. Really though - you don't need 0.25" in a 6" knife even if folks try to convince you it is cool to have. If you do go for a 0.25" chunck of steel then make sure to have it convexed to decent sharpness. Horton is doing some excellent super thick stock knives with a really keen convex edge. I really didn't care for my Scrapyard SOD at first until I did some mods to it (removed the ramp and guard and convexed the edge). Now I find it much more versatile. I used to only use it for chopping but now I can get a hold of it the way I like for more fine scale stuff.