I don`t know how much KB will raise the average attendance and you shouldn`t judge by this since the Greek league has very bad and very small gyms. But it`s 1000000% sure that every game Olympiacos will play will be sold out both in Greek league and euroleague and packed many hours before tip off.
Players of lower caliber where literaly worshiped like Gods.
Living in Greece will be an unbelievable experience for any top player. Bballwise of course Euroleague and the Greek championship can`t be compared to the NBA in any level.
I can`t understand why a top NBA player in his prime would move overseas. There is a big salary raise but it`s not like KB earns 1 mil/y and wants to get 2. Top players are already paid big
For a 35year old KB would be a good choice though.
UPDATEAfter talking to my olympiacos connections

it seems that the rumour is not 100% true. The owners do think about offering a contract to an NBA top star but that`s it. First they want to see how the Childress experiment will work out and then move accordingly. The "83m $ rumour" has started by Italian press and olympiacos` ownership is a little annoyed by this.