For the cheapest, get into DIY mixing.... check out the subforum here. I'm looking at getting enough unflavored nicotine to last me a couple of years for about $100.
I vape about 25mg stuff, and go thru 2-3 ml per day. Cignot has very reasonable prices (they're my first supplier, and shopping around, they're a really decent priced one-stop place)... 20ml at 26mg for $9 means about a buck a day. Get backstocked so you're not in "worry mode", then take some time and research Do It Yourself mixing
