During the session Architectural and Technical Challenges with SOAP at the OOP 2007 conference, someone asked me the question: "What is the overhead in your opinion in using SOAP over traditional RMI communication?" It is a hard question, because RMI comes with Java SE almost for free.
So I tried to be honest and answered: "about 30%". After my answer a developer from the audience answered the question again with his estimations: "not true, in out project we have about 70%-80% overhead". I became curious and asked him, why he uses SOAP then. The answer was funny: "Because of strategical decisions". After the session we discussed this issue with the participants and it turned-out, that in most of the projects there is Java on both sides (client and server) - which is really funny, if not crazy. You have an java-object, then you serialize it over ascii, to get it again, on the other side :-).
So sometimes decoupling can be really expensive: to build, as well as to maintain.
NEW: Online Workhop Effective WebApps without Frameworks is also coming to: MUC Airport.