Money is only earned from the missions as you play right? any points from kills is pure xp.
Bit annoying but maybe it's to stop farming.
You get money by completing the objectives in rounds 3, 6, and 10. If you were farming gold, for example, you could just suicide after finishing the objective on wave 10 to save some time.
In gold, round 3 nets you about 10k
Round 6 gives 20k
Round 10 gives 45k
My times for failing after round 6 are still faster than I clear silvers so if you're good enough for silver but can't finish gold all the way, gold is still a bit faster.