It's crazy how far the league has come, but the last time there was an expansion team was '04-'05 with the Bobcats, which was also the last time the Celtics sold.
The Bobcats paid an expansion fee of $300m which was agreed to in Jan '03. and the Celtics just sold for $360m a few weeks earlier (Dec '02). I believe the previous two teams to sell were the Mavericks in Jan 2000 for $285m and the Nuggets in July 2000 (the Nuggets were sold in a package deal with the Avalanche and Pepsi Center for $450m, but Forbes says the Nuggets portion was $202m).
So back then, you had an expansion team going for more than what established teams went for in larger markets a few years earlier, but less than a premier franchise. Following similar math, if the C's sell for $6b, and considering the Suns and Mavs recently sold for $3.8 to $4b, it makes sense for an expansion team at $5b (and of course the NBA will try to get more if possible).
It is hard to fathom buying a brand new team for $6b when you could just buy the Celtics. But if the Celtics are saddled with bad contracts, bad arena deal, bad RSN deal, etc. then I guess it could make sense if you're buying a clean slate with more growth potential.