Lottery!

[Osiris]

you only get more cash on siege if you have a large group of clan mates helping to keep a multi. Its my experience that siege teams/maps are far more balanced and as a result i find my avg multi lower and i get far fewer long x5 streaks (unless your raping the valour)

[Kafein]

Playing naked + best heirloom point merchant = heaploads of cash.

Also I think in siege it is more viable to use very light armor (and reduce your upkeep) because of the frequent spawns. Your life isn't that important.