Make sure you check the grade (don't know if that is the right word) of the steel. I thing the lower the number, the better the steel..but it could be the other way around. From what I've noticed the money for the cheaper knives is saved on the sheath. The knife may be great...but the sheath that holds it is basic, cheap plastic, and likely to crack in your dive bag. Since most knives and sheaths are sold as a set, make sure you take a good look at that.
Also, the straps are to be closely examined (if they come with leg straps...another way they keep the cost down). Of course, if you find a knife with a qualitly sheath and good blade, but cheap straps, you can pick up a set of good straps for a decent price and will probably make it worth it.