There are caveats. From Google's protobuf file itself [0]

    // If the bid won the auction, this is the price paid in your account
    // currency. If the bid participated in the auction but was out-bid, this
    // is the CPM that should have been exceeded in order to win. This is not
    // set if the bid was filtered prior to the auction, if the publisher or
    // winning bidder has opted out of price feedback or if your account has
    // opted out of sharing winning prices with other bidders. For first-price
    // auctions, minimum_bid_to_win is populated instead of this field.
    optional int64 cpm_micros = 4;

I don't know if Google's bidder will populate this field. Even if they do send this data, there's no way you can tell if it's faked or not.

They could also maybe get around this by setting up a PMP for Google's own inventory thereby allowing them to accept bids that are lower than Open Market bids.

[0] https://developers.google.com/authorized-buyers/rtb/download...