Go to Welcome to MadVapes LLC and check out the piece parts they sell for mod building. You can buy a variable voltage mod already assembled for $35. You'll need 2 14500 IMR batteries and a charger. That will add about $20-$30 to the total cost. They also sell regulator chips, miniature digital voltmeter modules, and atty connectors, as well as all the other parts you will need to assemble a good mod that looks store bought.
Variable Voltage Box Mod
Variable Voltage Box Mod