From 500 cattle produces 1,200 kWh/day
The biogas produced by the Penn State digester is approximately 60 percent methane and 40 percent carbon dioxide. This means that the gas has 60 percent of the energy in natural gas or about 600 Btu per cubic foot (22 MJ/m 3). Biogas. The biogas produced by the Penn State digester is approximately 60 percent methane and 40 percent carbon dioxide. This means that the gas has 60 percent of the energy in natural gas or about 600 Btu per cubic foot (22 MJ/m 3). The methane content of the biogas will fluctuate according to digester conditions. If controls for NO x emissions are added (another $90,000 in capital costs), the levelized cost of electricity goes up to about $0.077 per kWh. The most likely scenario for California is an anaerobic generator with NO x controls and co-generation, which gives a cost range of $0.062 to $0.077/kWh. Based on the operating costs of several of the Swedish biogas upgrading plants, we projected that, at a very large dairy (8,000 cows) or centralized facility, the cost might be as low as $8.44 per 1,000 ft 3. Table 8-10 compares our estimated costs for producing biomethane to current prices for natural gas. “Feedstock costs typically account for between 20 percent and 50 percent of the LCOE, but they can be even higher.”. Operations and maintenance (OM) costs can make a significant contribution to the levelized cost of electricity as well, accounting for 9 to 20 percent of the LCOE for biomass power plants. Fixed OM costs typically range from 2 to 7 percent of installed costs per year for most biomass technologies, with variable OM costs of around one-half a cent per kW hour (kWh). Landfill gas systems have much higher fixed OM costs, which can be 10 to 20 percent of initial capital costs per year. Operations and maintenance (OM) costs can make a significant contribution to the levelized cost of electricity as well, accounting for 9 to 20 percent of the LCOE for biomass power plants. Farm will buy energy from PG&E, and will be credited by PG&E at a rate of --50% of rate PG&E charges per kWh-- per kWh produced on farm source: RCM Digesters, www.rcmdigesters.com A 70kW generator operating on the biogas from 500 cattle produces 1,200 kWh/day (438,000 kWh/yr). Total Cost: $387,500. [Note: Energy consumption comes from www.RCMdigesters.com, (also consistant with PG&E 400-700kWh/day).] Assume farm purchases energy from PG&E at $0.1093/kWh, electricity costs/yr: (0.1093(rate) x 182,500(usage) = $19,947.25/yr) Routine daily maintenance of farm (checking machinery, cleaning up manure) is 1 hr/day. Capacity costs. According to the California regulatory agency study, the capital cost of a new integrated gasification combined cycle (IGCC) coal plant without carbon capture and storage (CCS) is $3,087 per kilowatt (kW), and the cost of a new IGCC coal plants with 90% CCS is $5,127 per kW, both in 2008 dollars.  Each cubic meter (m 3) of biogas contains the equivalent of 6 kWh of calorific energy. However, when we convert biogas to electricity, in a biogas powered electric generator, we get about 2 kWh of useable electricity, the rest turns into heat which can also be used for heating applications. 2 kWh is enough energy to power a 100 W light bulb for 20 hours or a 2000W hair dryer for 1 hour. What happens to the waste after digestion? Despite popular belief, the amount of waste going in the digester is almost equal to the amount coming out. Basic Numbers. The University of Guelph's anaerobic digestion calculator estimates the capital costs for a larger anaerobic digestion system to be $3,000-$8,000 per installed kilowatt (kW). Smaller systems (less than 100 kW electrical capacity) may cost more To produce 1.0 m3/day of biogas, which typically comprises 60% methane and 40% CO2 and some trace sulphur, one requires the black water from 15 to 20 people, or the manure from three cattle, or 10 pigs, or 1,000 chickens. That cubic metre of gas is sufficient to power a two-plate gas stove for a couple of hours. 
http://extension.psu.edu/natural-resources/energy/waste-to-energy/resources/biogas/projects/biogas-from-manure http://www.suscon.org/cowpower/biomethaneSourcebook/Chapter_8.pdf http://biomassmagazine.com/articles/8344/global-costs-of-biomass-power http://biomassmagazine.com/articles/8344/global-costs-of-biomass-power http://www.appropedia.org/Economic_analysis_of_a_biogas_digester http://www.sourcewatch.org/index.php/Comparative_electrical_generation_costs http://www.electrigaz.com/faq_en.htm http://www.omafra.gov.on.ca/english/engineer/ge_bib/biogas.htm http://www.esi-africa.com/sa-not-using-its-biogas-potential/
biogas cost per kwh