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Gravity budget : 100 million dollars.
Indian mars mission : 72.9 million dollars.

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The total length that is to be traveled by the satellite is around 680 million kms, and the total cost is 73 million dollars.

so cost per km is 73 / 680 which is equal to 0.107 dollars i.e. Rs. 6.4

Though it's a bit irrelevant it's fun to note that it is cheaper than any auto rickshaw or taxi.