I mix at 6 mg, 35/65 or as close as possible to that, and a 30ml costs me an average of $1.10.
I generally don't buy bases in large quantities, but have recently bought a gallon of VG from Heartland, just because vapocalypse.
I've also bought close to a liter of pre-mixed Kilo juice plus about 10 other flavors I've found at various places, so I'll probably use that up first, beford I mix much more. Point being, I really won't know the age of the pre-mixed juice and I want to use it before it gets too old to vape.