Interesting data, and I know you understand how the draft works, so this isn't directed at you so much as a general cautionary obsevation. Each draft slot has a designated salary range associated with it. A guy like Nick Johnson, who is a 6-2 tweener guard, had no chance of being drafted higher than he was--so leaving was smart.
Grant arguably could have gone higher had he come back and been the featured cog [admittedly, no guarantees]. Which means that the salary he got might have actually been significantly lower than what he would have gotten if he'd been drafted in the middle of the first round. In one year, he could then have made as much as he will make in two years of a 2nd round salary--and be better positioned in terms of those third year earnings.
So yes, anyone on the board who suggests that they would turn down $800K are probably not being honest. But there is an opportunity cost to leaving early if you slip into the second round. A rather significant one, if you are sacrificing eventual first round money [especially potential lottery - mid round money].
Which is exactly the situation that McCullough his facing. There are no guarantees--that's a given. And everybody knows that sites like NBAdraft.net are awful. But let's just use that as a starting baseline. They have him as the #5 pick in the 2016 draft. Who knows how accurate that would be. Hell, knock him down 10 spots, and then take a look at what THOSE salaries look like compared to the ones you present above.
Grant couldn't have been luckier. He made a poor business decision, disastrously tumbled into the second round, and then managed to land on his feet in an advantageous situation--on the worst team in the league, on a lousy roster that he could make, on a team looking to amass young / cheap assets for future trades, on a roster where he could actually get PT. If he'd been drafted by half the other teams in the league, he wouldn't have gotten out of summer league, let alone making the team at all; he'd be in the D-league making $25000 desperately hoping for a 10 day contract opportunity to hopefully prove himself. Instead, he got a TWO YEAR GUARANTEED CONTRACT as a SECOND ROUND PICK and made the Sixers roster. Very atypical. Practically unheard of.
I'm very happy for Grant, but let's not highlight that as an example of how the draft works favorably. He couldn't have gotten more lucky in terms of the team / situation--he stepped in a pile of sh/t and came up smelling like a rose.
McCullough might get drafted in the second round. Then again, he might not. I've heard that NBA GMs weren't enthusiastic about him potentially coming out, and he did anyway. So... unless he gets drafted in the first round [unlikely], or gets drafted by a team that is willing to stash him and allow him to make the roster [unlikely to a degree], he might end up being the anti-Grant circumstantially and find his NBA career over before it even begins.