I really don't know except they are popular. If you notice how most of them are made it is just a hollow tube of metal with some basic small wring with a small switch. How much could this cost to make. There are a couple that come with a battery but if you buy an extra battery it cost 22.99. One of the advantage of a mod is to have a longer lasting battery. The 3.7 volt does not give any more vapor that a regular ecig the battery just last longer. I'm not sure about the 5-6 volts although I have ordered one. I hope it give more vapor and a better throat hit but I'm not sure this will happen. Some offer a personalized paint jobs and some are suppose to have brass on them but lets face it they are very overpriced.