Because of the cost of isotopic chemicals and heterologous proteins to produce, an economical ¹⁵N/¹³C isotopic labeling method is critically needed. Four protocols have been tested for the expression of Ovine interferon-τ in Pichia pastoris. ¹³C-glucose in place of ¹³C-glycerol as well as the need for ¹⁵N/¹³C-sources were evaluated during the growth phase. Sequential addition of ¹⁵NH₄Cl and ¹³C-methanol were also evaluated at different ratio. Our results demonstrate that ¹⁵N/¹³C isotopes are not required throughout the initial growth period but are necessary at low concentration a few hours prior to the methanol induction period. We have evaluated the cost of the use of isotopes ¹⁵NH₄Cl, ¹³C-glucose and ¹³C-methanol in our optimised P4 protocol conditions. The cost was one-third that of the standard method using ¹⁵NH₄Cl and ¹³C-glucose throughout the entire growth period and was even lower using ¹³C-glycerol.