The main difference between a dirt cheap cog, and anything else is the dirt cheap cogs are stamped out of sheet metal rather than forged. A forged cog is stronger, has a better tooth profile and rounder. A really nice forged cog is round and has a better tooth profile than an average forged cog. If you need anything 16 or under get a Dura Ace cog, over that I am not the guy to ask.