since biogas spent slurry are very good organic fertilizers, you can use the opportunity cost of a choice to calculate it otherwise you can use shadow pricing

I think using a cost factor per litre of biogas produced.

If you want to connect a price to the slurry leaving the plant you can measure the NPK (Nitrogenes, Phosphor, Kalium) of the digestate and compare it with the price of each respective nutrient and get a price per m³ effluent.

Addtionally you can value the price per kWh electricity paid in the country of interest by the electricity provider.

I would recommend putting both in an excel and calculating on daily basis to have the daily cashflow of the plant.