Theorien über den Mehrwert
The concept of production prices is introduced and elaborated systematically in chapter 9 "et seq." of the third volume of "Das Kapital", although it is already referred to in earlier texts by Marx. The first significant discussion occurs in the "Grundrisse" (1857-1858), followed by numerous references in "Theories of Surplus Value" (1862-1863), letters by Marx to Engels of 2 August 1862 and 30 April 1868 outlining his theory, the "Resultate" manuscript (1863-1866), "Capital, Volume I" (1867) and "Capital, Volume II" (1865-1877).