Actually, low resistance is 1.5 - 1.9 ohm.
Standard resistance is 2.0 - 2.4 ohm.
High resistance is 2.5 - 3.0 ohm.
Low resistance will give a warmer vape. Standard resistance will give a cooler vape. High resistance is for variable voltage/variable wattage devices that put out over 3.7 volts of power to the heating coil.
I personally recommend single coils over dual coil cartomizers. The duals require at least 4.5 volts of power to really shine, and they drain a battery faster than single coils do.
FYI, "DCT" stands for dual coil tank. It is terminology that gets lumped into that Q-Tips are cotton tip applicators and Band Aids are adhesive bandages. The reason that I bring this up is that many people believe that you must use dual coils in a DCT tank. Nothing could be more wrong. Many, if not most vapors, use single coils in DCT tanks.
A cartomizer can be expected to last anywhere between 1 -3 weeks in a tank. Factors like dark or thicker juices will take a tougher toll on cartos and cause them to not last as long. When the carto's flavor drops or the pull becomes stiff, it's time to replace it.
As far as expenses with replacing a cartomizer....I spent over $5 a day on cigarettes and thought nothing about it, it's nothing to me to spend $2 every 1 - 3 weeks on a cartomizer in comparison.