Welcome, @nishikant bilaskar, to the community!
You mentioned two somewhat vague requirements: very low power and low cost. If I equate very low power with Ultra-Low Power, there is a clear price winner: the STM32L010F4P6. Maybe it already fits into your project?
Otherwise, you would have to list a few more requirements so that the selection can be better narrowed down. Alternatively, you can filter the Ultra-Low Power STM32 yourself on the website.
Hope that helps?
Good luck!
/Peter
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