How do I do a cost-benefit analysis for picking the to SCRUM/Agile trainer ?

I guess you need to visit the famous websites that offer scrum training. You can go through these and look out for the cost factor... but as far a benefit is concerned, it totally depends upon the trainers and the kind of material the company lets you take back..So even after you are certified, you would be able to look at the material and apply it confidently at your work.