The price of API access per million tokens, in dollars, by the cheapest offer across providers. Input and output are folded into one number: output is usually several times more expensive, and ranking models by the input price alone would put something other than the cheapest on top.
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The price is the lowest among the model's providers, base tier, without batch or discounted rates. Input and output are shown separately on purpose: for most models the output costs several times more than the input, and the final bill depends on which of the two your task has more of.