Using a BRMS (BRE) language for initial app development can easily improve developer productivity between 10-20% if you're using a good tool and an experienced team of KEs and BRE developers. And that's just for your first apps. As you build more apps and reuse more code, that number should go up.
But the real power of BRMS/BRE becomes obvious when it comes to maintenance: Companies should plan on and expect at least 50% reduction in change time and maintenance costs when changing rule-based applications.
If you're not getting between 50-75% reduction in maintenance costs using BRE/BRMS, you're either doing something wrong, you're using the wrong rule engine, or your BR harvesting/architecture/engineering methodology is not workling.
It would be nice to hear the BRE software vendors jump in and tell us what ROI their customers have experienced. I've been designing and building rule-based BRE/BRMS apps for a long time, and I've seen many companies hit these 20-50% improvements over and over again since 1988. I've also seen a few miss the mark for the reasons above.