While making 100M is no easy feat in Runescape, it can be done.
Icey Dan1 has an excellent video for those who are unfamiliar with how to exploit Runescape skills for money purposes.
Thersmg has an excellent set of 1gp-100K, 100K-400K, etc... Personally, I recommend starting here.
My current personal favorite method is collecting whiteberries in the wilderness. They sell for 1200gp each (so that's about 28k every 7 minutes, or about 200K per hour). If you have level 60+ or so combat (to avoid Revenants), this is your way to make money.
Of course it's up to you. And anyway, remember:
Just have FUN!