Feedstocks characteristics and their provision cost can have a significant impact on the cost of methane production in an anaerobic digestion (AD) facility. This study investigated the impacts of changing grass silage characteristics, grass silage and cattle slurry provision costs and their binary mixing ratios on the cost of methane production from an on-farm AD facility. The feedstock provision cost contributed about half of the total cost of methane production when the AD facility solely operated on grass silage. The total cost of methane production from mono-digestion of cattle slurry, compared to grass silage, was 87% higher when it was supplied free of cost and was 42% higher when a gate fee of euro70 t(-1) total solids was charged. For co-digestion of grass silage and cattle slurry, the total cost of methane production progressively increased as the proportion of slurry in the co-digested feedstocks mixture increased. Antagonistic and synergistic methanogenesis resulted in a corresponding 6% higher and 5% lower total cost of methane production during co-digestion, at silage:slurry volatile solids ratio of 0.8:0.2, compared to the binary mixture without these effects. (c) 2018 Society of Chemical Industry and John Wiley & Sons, Ltd