Hi Mathew,
The Certification is Free of cost, but we have certain pre-requisites for that. We need to undergo training to be elligible (Register) for writing the EAI Developer Certification Exam. The fee for the Training can be found on www.webmethods.com . I think you can find all the details there itself.
Certification can be a valuable thing, but having confidence in knowing what you are doing is much more important, in my opinion. I have yet to run into a client that required me to be certified in wM. What they are looking for is competence; if you don’t have certification, but have and demonstrate skills, you will probably be just fine. Sure, the certification helps to validate those skills, but no more than the first five minutes of a wM candidate interview.