Its quite project specific. Depends on where you are, what waste material you want to use, what you want to use the energy for etc etc.

It depends what scale you are talking about, I don't know much about domestic systems and bet they are very few and far between and are probably home made affairs. Farm sized systems take a lot of work to manage. It is basically a full time job adding feed/waste everyday and making sure the ration is set up correctly as the volume of gas produced can vary widely depending on what you are putting into it. Very much like feeding a cow except you are trying to feed the microbes the best diet to maximise the production of methane. I think small systems start at about 250kw but go right up to multi-million pound MW industrial sites which make a lot of their money from gate fees received for taking food waste from supermarkets or councils that would otherwise go to landfill. I think as the number of plants has increased gate fees are dropping due to finite supply of suitable waste food so the economics can be uncertain. Farm based systems usually have a certain amount of animal manure added but the bulk of gas production will come from crops grown specifically for the plant such as grass or maize silage. They can work very well with a large mixed farming operation especially if the farm can utilise the waste heat generated by the gas turbines, to heat chicken sheds for example. The waste digestate is also an excellent fertiliser which can be applied back to the land the crops are grown on.