Hell's Forge happens to make the best nail knife out there right now, and they make it for the best price.
This nail knife, by Hell's Forge, is the cheapest on the market. Made out of a 1/2 inch steel bar, this nail knife is extremely reliable. This knife would be best used for display, cosplay, and war reinactments by dulling the blade. There is not that much else to say except for the fact that it rusts quite easily due to it being an un-coated piece of steel.
I give the Hell's Forge French Nail Knife a 9/10 for being sharp, strong, and reliable. I mark off the point because it can rust easily if not oiled well after use.