I think you are referring to WebSphere Message Broker or WebSphere ESB compared to open source ESB offerings such as Mule and ServiceMix. Cost is of course a main driver to choose for an open source ESB. But other advantages are:
Flexibility: you can easily extend a feature of the open source ESB to match your specific requirement, with the source code available. For IBM product offerings you will have to wait for the next version or raise a PMR.
Community: open source ESBs have a great user and developer community behind it to answer your questions promptly and to implement new functionality on your request (if there are enough other users wanting this).
Lightweight: IBM product offerings are large and complex, so you would need extensive training and you have a lot of installation work ahead of you. Open source ESBs are really lightweight. You download them and within a few seconds you have started the ESB container (try that with an WebSphere ESB).
Just a few advantages for an open source ESB.
Author of Activiti in Action and Open Source ESBs in Action