The cost of training for these certificates varies greatly depending on your abilities and how long it takes you to acquire the necessary skills. The CFI rating is fairly involved and generally requires more time and money than the CFII and MEI add-ons.
For answers to specific questions including time, age, and training requirements for these ratings, refer to the Federal Aviation Regulations, or give us a call and talk to one of our flight school instructors. We are happy to help you.