Resistivity rho, thermopower S, and thermal conductivity kappa were measured simultaneously on a set of CoFe alloy films. Variation of the Co content x(Co) allows for a systematic tuning of the Fermi level through the band structure, and the study of the interplay between electronic and magnetic contributions to the transport coefficients. While band-structure and magnon effects in rho and kappa are rather weak, they turn out to be very significant in S. A decomposition of S into Mott and magnon drag contributions results in a systematic evolution between the two limiting cases of pure Fe and pure Co. At low temperatures, we find an interesting sign change of the curvature of S(T) that indicates a corresponding sign change of the magnon drag.