Frankly, I don't think they are worth it. My buddy bought one and we used it to try to find a doe his son shot. The doe died intall grass in a bog. The think couldn't locate her from a distance of 100 ft. We ended up tracking her down the old fashioned way.
Have you ever tried using Starlight Bloodhound? That stuff really works. Makes the blood glow in the dark even after a couple of days and being rained on.