$30 each
 
This model has an adjustable voltage of 2.7 - 3.6 with a button that locks and releases the oil cartridge for storage in your pocket or handbag and flips open for use. It has a corner piece that attaches to a keychain easily! Flip it open for usage then flip it closed for storage! Attached standard USB cable for charging.
Some more viscous liquids may require a higher voltage, while more diluted cartridges may require less, as well as allowing you to adjust how strong you'd like your draw to be. Excellent and versatile (unlike some that aren't strong enough to draw or that burn your oil leaving it tasting unpleasant).