From what I've seen, a first round pick usually buys you about 5 million in savings.
I think you may be a bit outdated here. Look at the Philly / OKC trade:
December 8, 2020: Traded by the Philadelphia 76ers with Théo Maledon, Vasilije Micić and a 2025 1st round draft pick to the Oklahoma City Thunder for Terrance Ferguson, Danny Green and Vincent Poirier. 2025 1st-rd pick (PHI own) is top-6 protected Philadelphia also received a trade exception from Oklahoma City.
A second rounder, the rights to a second rounder drafted back in 2014 who may never join the NBA, and what is likely to be a non-lottery pick in 2025 for a useful starter, a deep bench player, and a small expiring contract. In that deal, Philly cleared at least $70,000,000 in guaranteed salaries from its books.
Micić is a very tough player to evaluate in terms of his value. He's the best player in Europe, and for 2-3 years he has sounded like it's his last year in Europe, only for him to stay there. I'm not sure what OKC's valuation of him is, but I don't think the C's could get his rights simply by sending the Thunder cash.
Also, Philly cleared less than $70 million. They cleared closer to $50 million when you take into account the incoming salaries. Anyway, the $30 million in savings the C's got for mid-teens pick they gave up is in line with what Philly gave up for their own savings.
I don't think so. You're ignoring the difference in value between the #16 pick and the lesser picks that Philly gave up, while also ignoring Danny Green. That's also without considering any cost sayings from buying out Kemba.
I'm not ignoring Danny Green. He was a player the Sixers put into their rotation, same as the Celtics put Horford in their rotation. Both are/were overpaid relative to their production (Green made $15 million last year, and signed for $10 million this year), but still useful, so they aren't dead salary. Neither the Celtics nor the Sixers only cleared salary, they also received a player in return they found useful.
As for Kemba being willing to give back some salary to OKC -- that wasn't ever a viable option for the Celtics. Yes, Kemba cost OKC lless than he cost the Celtics, but that doesn't change what the Celtics saved. The C's couldn't carry $26 million in unmovable salary this year and next any more than they could carry $36 million in salary that no one was going to want without compensation, unless they wanted to blow everything up.
WRT to the draft pick, I'm not privy to front office decisions to know how Presti values a Top 6 protected pick five drafts in the future relative to #16 immediately. I think they're closer in value than you might suspect, because the chance that you get a top 10 pick matters a lot.