True. Which is why you would need many credits. For example, I get about 4.7 mil for 15 credits, which I think isnt worth it but if you want to go for it, hey, why not?
Dorm 18 gives you 46? mil Being LC gives you 1.3 bil I believe. Truthfully, when you don't have many dorms, stick to battles. Or you can volley, and then use up your stamina doing the smallest job, and save up all the credits. I get about 30-50 credits in 2ish weeks. By the time you get to your 20th-ish dorm, then it's a bit more worth it to use credits for cash, and by then you should have a wealth of credits to do so with.
But for the money worth.... I always do the one with the difficulty of difficult that give me the most money.