The aim of this work, as an extension to SAFERNAC model, was to establish economically optimum combinations of N, P and K application to Arabica coffee in the Northern coffee zone of Tanzania. The study was conducted in Hai and Lushoto districts between 2010 and 2012. Prices of nutrient inputs and those of parchment coffee were introduced into the original SAFERNAC model, which was used to obtain yields from a soil of known properties receiving different levels of input N, P and K from both organic and inorganic sources (ISFM). The costs of these were derived from experience in Northern Tanzania, while coffee prices were estimated to range between 1250 and 2500 TZS kg-1. The result was economically optimum N:P:K ratios that give highest net returns and value : cost ratios in situations of low, medium and high soil fertility. It was also shown that farmers’ decision to deviate from the optimum and the allowable level of such deviation, depend much upon the prices of nutrient inputs in equivalent terms. In the medium-fertility situation (which applies in the study districts), the highest yield increment was noted with the maximum amount of N and P. The optimum application rate was 310 kg N and 200 kg P per ha, where the profit margin (the gap between gross returns and costs) is highest. This is an indication that soil-available K is likely to suffice the needs of the crop for optimum productivity, but this is largely dependent on the K fluxes in different soil types. The optimum rates were tested with actual soil data in the two study districts, against 5 tons of farmyard manure and a combination of the two. At both the coffee prices of 1250 and 2500 TZS kg-1, ISFM intervention (combination of organic and inorganic inputs) was more profitable than the other options, while coffee production showed to be more profitable in Hai than Lushoto.