I often get asked about the decision management market - especially about its size. Unfortunately there is no one number for this. People track the size of the business rules management system market (IDC is about to release a new report, for instance) and most of that is really decision management. Similarly there is reporting on the size of the analytics/data mining market and a solid core of that is decision management also. Optimization technologies, like CPLEX and Dash, are almost 100% decision management. Taken together this is several hundred million.
However this is just the beginning. The money spent on decision management applications is very significant. Banks and insurance companies, for instance, spend hundreds of millions on products for credit line management, fraud detection, claims, underwriting and more. Much of this is decision management spend also (though some is really workflow). In the CRM space, Chordiant's products are increasingly decision-centric and companies like Oracle are selling "best next action" modules that are clearly decision management by another name.
As you get out into more specialized products there are even more with everything from scheduling engines to recommendation engines. Some of these are standalone and some are modules within larger products but there are more and more of them every year.
Sadly no-one has really tried to gather this data and put a total value on it but there is plenty of activity if you think about decision management in its broadest sense.