I've been doing main missions when I want expensive Argo upgrades for that reason. Even merc missions have some pretty good boosts from optional objectives. I had an escort mission where you had to protect a VIP in one of four APCs. After going 4/X salvage the mission payout was 67k. However I had optionals for making sure all APCs survived, wiping out the enemy and wiping out their reinforcements. Completing all those boosted my money payout to 250k I salvaged two mechs off the mission as well. So I ended up making 1mil Cbills off of the mission before I sold off the basic weapons and heatsinks. The max money payout for that mission was like 700k.
edit:
@OneofOne on that mission I just rushed a medium mech to kill the two trucks while I had my other three mechs hump the hill at the southern central portion. Used the medium to just sprint around distracting two enemy mechs while the rest of them tried to come around the hill getting alpha striked one by one.