And this is the problem. It'd be fine if, when people stack 20 missions each to kill 12 skimmers each, that they *actually had to kill 240 skimmers*. The biggest reason for the extra cash for kill missions isn't necessarily the risk, it's the inconvenience. If I'm taking a kill mission, it's pretty damn risky to also be taking cargo/message delivery missions. You get popped, you fail all them... same same for assasination missions (too risky to combine with cargo hauling). And you don't see people "stacking" assasinations them because lo and behold, just like cargo delivery missions, you actually have unique objectives to meet.
So as a standalone mission, one run to kill 12 skimmers for 2m is fine. The problem is stacking though... which means the reward gets nerfed all to hell... and then people go "Oh look, a message delivery pays way more than killing skimmers, that's out of whack!" and then they get nerfed too and then we're back to "Hey, haul 200t of cargo and I'll polish your shoe for you".
For what it's worth, an FD employee (forget which one, maybe the missions guy?) did say they were looking broadly into the games economy, basically, because progression doesn't scale. So honestly, be ready for credit earnings to go up before they go down...