The core can certainly be kept. Nice short answer wasn't it
Bogut will have to resign for no more than 11 million a year for no more (and no less) than three years and even that depends on how many games he plays this season.That's three million less than he gets this season. Lee, who is good for us and and who I think will honestly do bit less since the young guys are developing and getting better, will have to resign for no more than 10 million a year himself, if he is still here at the end of his contract, and at the age he will be, 10 million might be too much even and the length will certainly be no more than two years, so maybe that's the main point in wanting to trade him now, as his value will only drop from here on. More money to use on resigns.
I think Klay might not get that much better that he would demand more than 11 million himself a season. The one that will maybe cost the most is Barnes and that's only if develops like he is expected to. By that time, Iguodala will be coming out of contract and he will be too old to get more than 8 million himself and that also depends on how he is performing by that time.
That's an average of no more than ten million for the six main guys and that would leave some ten million to fill the rest of the roster, to stay under luxury tax, which is enough money.