I find the whole proposition unlikely, but the Raiders' behavior towards him gives me enough pause to consider it (and it's so tantalizing that I can't help myself). It's not uncommon for teams and players to be at something of an impasse by the beginning of camp. But it is unusual that they haven't had contact with his camp since February and that Gruden hasn't met him. Those details suggest to me that maybe there's something more than "hard ball" going on here.
As for the last question, that all comes down to what he costs to acquire. The contract is going to be around 5 years, 100 million. You can give or take a little bit there, but that's the ball park. And that part is easy for me-- if you have a chance to pay market value for an elite player at a premium position, you do it. If they included Matthews in the swap, as I suggested originally, that frees up 11.37M in space this season which would help fit his new contract in. Beyond this year, the Packers have 38M in space with more available in potential cuts (Bulaga, ~7M; Tramon Williams, ~5M). Now they also have to fit in Rodgers' new deal, so it's probable that both Rodgers and Mack would mean they couldn't do much else of note on the open market. But in my opinion, that's more than OK. You're getting a guy who is so far and away better than what normally hits the market as it is.
The real question to me (assuming the unlikely scenario that the Raiders are willing to do him) is what he will cost to acquire. Chandler Jones was not quite as good as Mack after year 4, but he was close. He was similar in age, position, and production. The Patriots flipped him to the Cardinals for a 2nd round pick and Jonathan Cooper (a failed guard). So I don't think a 1st and Clay Matthews is crazy. It's both a better pick and a better player than what the Pats got, so that allows for the fact that Mack should cost more. The Raiders would be saving more space than they'd incur with Matthews, and he would help replace that important production. They also seem to have an affinity for veterans and Packers.
So in that scenario, I would consider it a worthy, worthy gamble. And I don't even see that much risk. Mack is an established, durable, elite player. Barring a disastrous injury, I don't see the downside.