Your gold should naturally change depending on how fast you level your girls and how quickly you obtain your girls. Ever since they started coming out with reissue events and I've had double the amount of 5star girls to level, I run out of gold instantly and never been above 500k. Its only recently after I finished leveling 4-5 of them in two weeks that I've been able to build gold up again. It got so bad for awhile that I started saving my level ups for Saturday so I could spam more runs through the gold daily.