Hello, hope you are doing well, to get mbbs seat in aiims NRI seat you need to appear for the NEET exam. AIIMS have only 72 seats among them 50% are for general and 50% are for reserved category. If you are generally your rank should end with 37 or 38, it means that you should be in the top 50. The fee structure for a resident Indian and NRI will be the same in AIIMS. The tuition fee structure in AIIMS is surprisingly very low, it's around 1700 all over the year.

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Wish you good luck!:)