Here's my method for upgrading the silverback:
Mutators: No ammo pickups, Must active reload, Friendly Fire (FF is key, any other +10%s will work)
This setup gives you 125% of money spent towards silverback upgrades.
Start wave 9 on trenches and buy the command post at the top by the one shot spawn. Do not spend any more money. Camp up here and rack up enough money to buy the silverback ($12,500). Let the last enemy kill you to repeat the wave over and over.
When you can finally buy the silverback, take it up top with you and use it for the waves, but be careful to not let it get destroyed, because if you don't have enough money to buy it back in the next intermission, you'll need the whole $12,500 again to buy it again. If its health goes in the red, park it in the corner and finish the wave on foot.
During the intermission, damage the silverback and put all your money into repairing it. Damage it by meleeing it to conserve ammo.
With this method I can rack up about $7000 in 3-4 mins once I start using the silverback. With a 125% multiplier, that's about 8750 towards the silverback every wave.
EDIT: turns out shooting the silverback instead of meleeing gives you some extra money
EDIT 2: Just got level 3 in about 2.5 hours using this method. I started out as a low level 1, maybe about 1/4 of the bar filled.
Last edited by TheFalcon; 11-02-2011 at 06:50 PM.