First of all let me make one clarification to your query. There is no such thing as a CMMI certification. What an organization receives are the results of a SCAMPI A appraisal that indicate the Maturity Level of the organization on the day the appraisal concludes. The organization is not certified. And there is no such thing as an SEI certification body anywhere in the world. What do exist are authorized SEI partners that are allowed to provide CMMI consulting, training, and appraisal services. Visit this web site http://partner-directory.sei.cmu.edu/ and you will be able to find the SEI-authorized partners in India.
The answers to your questions are highly variable depending on the size and scope of your organization and your geographical location. The best place to obtain realistic estimates is to ask several local SEI-authorized CMMI consulting and appraisal providers for their cost proposals, then you will have to handle the external costs. Internal costs really cannot be determined until you figure out how much work you have to do in order to implement the CMMI and prepare for an appraisal. Suffice it to say, your internal costs will most likely be greater than your external costs.