a few days ago i had like 200m. its too much in my opinion, and i didnt have fun playing because i could afford whatever i wanted. i sold my cash, kept 10m, and i enjoy playing alot more because i can make it all back and it gives me a goal.
Just joking If I were you, I would invest that money or use it to make yourself more cash. Flipping is a good idea, or you could invest a small portion in some bossing gear and go hunting for some valuable rewards!
Personally i would invest it into something you've always wanted? 95 prayer? or even pay a quester on here and such. Or like the majorty have stated, invest in some armour and boss hunt, can be worthwhile if you get lucky