Cheap Efficient Ways To Boil Water in a Survival Situation.

In a survival you use what’s available (usually wood).
others items must be brought with you (not efficient)
a small amount of salt (1 tsp) lowers the boiling point and the water will boil quicker
the most important (to boil water quickly. always keep the lid on the pot)
a small amount of veg.oil helps to keep the pot from over boiling.

Warm Water Before Cooking

Warm water will boil faster than cold water. Put your water in a dark container and sit it in the sun for a couple of hours before you need to boil it. A sealed reflective box, (basically a solar oven), will raise the temp even more. The warmer the water the faster it will boil.

How much alcohol depends entirely on where the stove is set at.

When you burn the alcohol on a stove, most of the heat is wasted - around 95% of the heat escapes as hot gas or lost radiation. To be efficient, you need to contain the heat so that it goes into the water instead of escaping.

Using an alcohol stove in an open, breezing area will require almost one US quart of alcohol to bring 1 US gallon of 5C ground water to a full boil at sea level. In a confined area, two US cups can boil one gallon. 

By putting the water-boiling container inside of a well-built stone chimney-style fire pit, you can boil a gallon of water with less than one cup of ethanol. However the entire alcohol stove heats up and creates a situation where the stove can boil and explode.

Personally I do not consider alcohol stoves to be good survival gear. They are EXCELLENT backpacking gear and in fact one of my preferred cooking tools for backpacking. 

It’s two totally different scenarios in my mind. When you backpack, you are one cell-phone call away from help. If you need to survive, then you do not know when help is coming or when you will find your way back to safety. 

It could be days, weeks or months. In any survival situation, having a good long-term ability to make fire from local wood and brush is much more important than carrying alcohol. So in addition to a few emergency 'wet weather' starters, I carry a thumb-size flint striker with my knife. 

Viola - ability to make fire from kindle shavings for decades and the wet-starters are there in case I ever need to start a fire in acclimate weather.