Ja will hit/cost the Packers more than he will for ANY team acquiring him.
This is where you are confusing yourself.
"Hit" and "Cost" are not the same things. In this situation, since you are talking about 2 teams and a current player under contract, you have to look at "New Cost" to each team, which is the money paid out to a player, whether its upfront or deferred. "Cap Hit" can be 2 different things. It can be "new money hitting the cap", "old money hitting it" or a combination of the 2.
If his contract doesn't change, JA's new
COST will be the same for the Packers, as it would be for any other team that they trade him to. JA's new "HIT", would also be the same for both teams. What would be different for the Packers and a new team, would be the "old money hitting the cap" of the Packers.
IF JA's 2025 salary is a "good deal" for other teams, it is because the Packers have already supplemented it, through deferrals. JA's contract is for $21M/year, over the next 2 years, a new team would pay less than that to him, because the Packers already paid him the difference. Basically, by trading JA they would be saying to the new team "Here, have this guy on a very reduced salary, because we already gave him that money, now we have to go report it on our cap."
Essentially, by trading JA, the Packers are saying that they don't want to pay him any
new money that is due on his contract. IF the Jags say "Wow, this guy is worth $21M a year and we only have to pay him $17M/year for 2 years, lets give up $8M worth of draft value for him."
Also and a completely different part of the discussion, by trading JA, you will be needing to replace his services on the Packer team, how much will that cost the Packers?