Depends very much on what you are growing. Most vegetables are very heavy feeders and without something providing nitrogen, few brassicas (cabbage family) will grow very well, unless you are lucky enough to have a naturally fertile soil.
The nutrients in a bag of compost will only last long enough to get your vegetables started.
There is a balance to be aimed for though, as too much nitrogen results in soft sappy growth which is vulnerable to diseases and pests, and too little just results in poor growth.
As someone who believes in organic methods, feeding the soil is more important than feeding the plants, so I tend to use 'feeds' which break down more slowly, such as blood, fish and bone, or if a 'quick' feed is needed, comfrey liquid or seaweed solutions.
You are right though that proprietary feeds and composts are absurdly expensive, when you can make your own feeds very easily by soaking fresh nettles or other materials in water for a week or two.

We have loads of nettles where the pony is, what do I do? Is it just simply to soak them for two weeks, how much water to how many nettles?

have you noticed the soil under your nettle bed is very fine ,even better for seeds than compost!