DIY e-juice!!!! You are in control flavor wise and its about 80 cents/30ml bottle. Even if you load it up with concentrates to make it stronger tasting or you use more nic in your juice you can still keep it under $1.50/30ml bottle.
I should not that those are the average prices Ive worked out based on the prices through my supplier. Your prices may vary but its still a lot cheaper of an option