I'd keep saving if I was you OP. Occasionally I'll buy a yellow or purple mod from the mod shop in hopes to get a good speed upgrade. Also I avoid seven starring a toon if I'm not gonna use them at that moment. I've been farming credits like this for a while and currently I never drop under 100million in credits. I remember what is was like to be dirt poor and having to rely on 600k Galatic war credits just to upgrade a mod or a toon....I never wanna be starved for credits again.
I rather hoard and save up for when the time comes that I need to use them instead of being broke.