The imperative of sustainable agricultural development, coupled with growing challenges related to climate change reaffirms the importance of manure and increases the demand for it. Due to the underdeveloped market for manure, there is a problem in assessing its economic value, requiring appropriate research on this topic. Thus, this research aimed to assess the manure’s economic value using the method of equivalent substitution. For this purpose, the chemical composition of manure was determined by standard agrochemical analyses concerning the content of the most important mineral elements—nitrogen (N), phosphorus (P2O5), and potassium (K2O). These elements ranged from 0.49–0.60%, 0.15–1.10%, and 0.70–1.07%, respectively, and the content of organic matter ranging from 20.88 to 27.00%. Subsequently, the cost of equivalent substitution was calculated taking into account market prices for commercial fertilizers and organic matter based on the cost of an adequate quantity of white mustard—Sinapis alba L. fresh mass. The average cost of equivalent substitution for manure was determined to be 28.60 USD/t. The results of the applied t-test indicate that there is no statistically significant difference between the market prices of beef manure and the calculated prices of its substitution (t = −1.4069; df = 12; p = 0.1848), which implies that white mustard green manure could replace the deficit in animal-derived manure. Since both animal and green manures unambiguously should be prioritized over chemical fertilizers, future studies will reveal which other crops are applicable for further increments of green manure to make up for insufficient availability of animal manure.