Cost basis for SpaceX pre-IPO equity sold after IPO

Apologies…I’ve looked, seen several different answers and still don’t understand. I worked at SpaceX before IPO and held RSUs that transitioned to Class C at IPO. Recently sold some of that equity and am trying to find cost basis for taxes. Is it the IPO price ($135) or is it based on the vested equity cost (varied because I acquired several lots at different dates). Held the RSUs for over 2 years. -Recipe106

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