Cheapest and most exact way to scrape small amounts from one or more accounts?

I've got one or two scenarios I'm wondering how to "perfect" in terms of costs.

I've got multiple accounts with very small amounts of a token, that however has SOME value.

How do I exact the amount of ETH I need to send to make the transfer of the ERC20 back out? No more, no less. I'm happy with answers where I can assume some set gas price, e.g. I'm willing to wait for a long time for it to be 20 gwei for gas.

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Thanks for any insight!