Yeah, the NFL seems to have a bit more widely accepted trade charts. In the NBA the trade value for different trades seems highly variable and dependant on the perceived drop offs at different tiers.Take these things with a grain of salt, but here are some models that attempt to assess trade value: http://nbasense.com/draft-pick-trade-value/4/jacob-goldstein-4
There are 4 versions of the model, here is what it would take to get #6 according to these charts:
Goldstein 2017: 6 for 9 + 42
Pelton 2017: 6 for 9 + 34
Pelton 2015: 6 for 9 + 50
Restifo 2016: 6 for 9 + 30
These are EV centric calculations, so not a true trade value.....but yeah I think they underestimate the cost from moving up to 6.....but it is a warning shot to show that the difference in outcome between 6 and 9 is not as big as you may think.
Some relevant recent draft day trades:
2020: Jarrett Culver (#6) FOR Cam Johnson (#11) + Dario Saric
2019: Deandre Hunter (#4) + Jordan Bone ($57) + Solomon Hill FOR Jaxson Hayes (#8) + NAW (#17) + two future second round picks
2018: Luka Doncic (#3) FOR Trae Young (#5) + future top 5 protected first round pick
Honestly, I think #6 for #9+#28+stuff (some seconds?) would not be out of the ordinary. It will depend on how much ORL wants who they can get at #6 versus #9.
To get to #4, it would probably take #9+#16+future first (not sure #28 is good enough).
When talking about pure value, independent from the actual prospects at those positions, you would have to pretty much never trade up for a player because by all those models, you would be overpaying massively. For example, 9+16 should probably get you to the 2-3 range, but in reality that's never going to happen. You would need to add a ton more.