Bang Dream is not a game that screams high budget, yet within 3 years it took over Love Live in gaming revenue.You basically HAVE to spend insane amounts of money to have even a slight chance of succeeding in the mobile space. Or be luckier than Fortuna & Tyche combined if you don't have that kind of money to spend.
Even games that have already found success need to keep spending fucktons of money on marketing to not fade into obscurity, or at least to keep the ball rolling.