A study of 30 COBOL products built by a large IT firm between 1984-1996 found that increases in CMM process maturity were associated with increases in development effort.

- Harter, D.E., Krishnan, M.S., & Slaughter, S.A. (2000). Effects of process maturity on quality, cycle time, and effort in software product development. Management Science, 46(4), 451-466.

To be fair, the article suggests the increased development effort was worth the increase in quality. However, I think each project team needs to decide how much they can spend on quality. The article goes on to state:

"It should be noted that the relationships between process maturity, quality, cycle time, and development effort are valid only in the ranges observed in this application domain (custom software development of an algorithmically intense system in a COBOL, mainframe development environment). It is possible that the effort and cycle time incurred to achieve considerably higher quality levels in a mission-critical software product could be significantly higher. ... Hence, the investments required to achieve extremely high levels of product quality may not be recovered by reductions in cycle time and effort."