Abstract Life cycle costs of co-digestion plant of cattle farm manure and locally available freshwater macrophyte C. demersum, marine brown algae F. vesiculosus, and marine green algae U. intestinalis; ratio 5:1) are analysed based on Latvian climatic and economic conditions. Biomass collection from nature and pre-treatment of biomass, biogas production, biogas treatment and utilization in combined heat and power plant are included in the boundaries. The weak points of scenarios are large capital investments, electricity sale price (and the application of feed-in tariff). As naturally grown algae and macrophytes are used, they are also sensitive to weather conditions each year as available amounts of biomass might change and decrease. Net Present Value is positive only for C. demersum with Internal Rate of Return of –14 % and Discounted Payback Period of 11 years.