This paper presents the results of a Grounded Theory study of how Software Process Improvement (SPI) is applied in the practice of software development, focussing on what is actually happening in practice in the software industry in relation to the adoption of SPI ‘Best Practice’ Models, such as ISO 9000 and CMMI. The results produce a picture of attitudes and perceptions in relation SPI best practice models, grounded in the field data, and reveals that many software managers reject SPI because of the associated implementation and maintenance costs and are reluctant to implement SPI models such as ISO 9000 and CMMI. This paper presents 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.