Brady is worth more, financially to both the Chargers and the Raiders than he is to the Pats. Consider the jeresy sales alone. However, the Raiders will have absolutely no problem selling out each and every game. Its Vegas, the casinos alone would buy up all the tickets.
The Chargers however, need Brady, they need something, they are co-locating with the Rams in their new stadium. The Chargers couldn't sell out a high school gym last year. No way they fill that stadium (without Brady). They need some sort of excitment. They have around $57 million in cap space right now, they can free up another $8.5 million by cutting Trai Turner. The have the Williams' and Kennan Allen, and a decent defense. Of course, that means two games a year versus the Chiefs, Raiders, and Denver, so making the playoffs would be tough, but they would be in LA (where his other son lives), sell everything on the East Coast, Giselle could make more money and Lord knows how much money the Chargers would be willing to put up for Brady.
The more I think about it, how quickly they jettisoned Rivers, this may be the Chargers plan. And the more I think about it from a Brady perspective, this might be the place he is looking to go.