Software Process Improvement (SPI) ‘best practice’ models such as ISO 9000 and the Capability Maturity Model Integrated (CMMI) have been developed to assist software development organisations by harnessing their experience and providing them with support so that they can produce software products on time, within budget and to a high level of quality. However there is increasing evidence that these models are not being adopted by Small to Medium sized Enterprises (SME) and primarily remain the remit of large organisations. This paper presents the results of a Grounded Theory study into why Irish SME software product companies are not using these SPI models. The key inhibiting factor found was the issue of cost. We discuss the findings in relation to cost of process and the factors affecting it, including bureaucracy, documentation, communication, tacit knowledge and organisational creativity and flexibility, and the associated impact on the adoption of SPI best practice models.