The aim of Cost Sensitivity Modelling (CSM) is to analyse the relevance of input data in weld cost or production investment calculations, and to show through different calculations how the final result is influenced by variations in input data in order to find the most important cost/profitability determining parameters. Today’s welding production systems are complex installations both in terms of technology, day-to-day operation, maintenance and investment. This is evident with processes such as Tandem-MAG and Laser-hybrid MAG welding. During the past decades, the profitability of investment is more in focus than, for example, production cost per metre weld length or produced unit. CSM can be applied in conventional weld cost calculations (cost/m produced weld) or in complex investment calculations. In this paper both cases are analysed to demonstrate the usefulness of the method. Two crucial questions arise as a consequence of CSM: how accurate is the input data used and which cost factor(s) are of prime importance? It is shown, for example, that deposition rate is not the only important cost determining factor in sophisticated welding systems. CSM identifies other factors which must be controlled in order to keep deviations in profitability from target value within acceptable limits. Implementation time in a new investment is one such factor.