I am trying to reply all the bullets in your post one by one.
- Some of the agencies (AtoS and some others) I have got in touch with to discuss this certification have suggested doing SAP SD or MM certification, keeping my experience in view. If I do it what is your guess how soon I could find a job with this certification.
for training and certification you can go to any SAP authorized institute , you can find many depends upon which country you are presently staying.
- What kind of salary will I be able to attract if I do get a job. Currently I am getting good salary by auto industry standards (around $15K). As per the information I have got the starting salaries in these roles are some times less than half of what I am getting now.
if you are already at $15K (1,50,000 US Dollar if i correctly understand) 
then you are earning much higer then a SAP consultant having a decade experience in SAP. Better look for management job. If salary is the criteria.
- Finally how fast is the growth in this industry? Let us say I do the certification spend some $500 and join a company with salary less than half of my current. After doing this, as per your experience how soon will I be able to reach or exceed the salary I would have been getting otherwise (assume I perform fairly well in my SAP role ;-).
refer the above one
- So finally should I go ahead with the certification?
Certification is optional , if you just have user level experience you may not be knowing all what comes within an module, i still recommend training as it give you all back ground about SAP and you can understand most of the master data piece to create a right foundation for a module.
Hope this will help you.
Thanks
Ritesh