Christie's terms specified the fee was not payable in ETH, only in government currency.

If it's going to another Eth address then it's as simple as sending a transaction. Fees are a problem these days but they could've sent it for as little as 1-2usd to as high as 30-100usd. Higher fees mean faster processing.

Sounds like a tiny portion of ETH's daily volume ($23B). I'm not sure if it would even noticeably move the price.