Newer Ironmen the best thing to do for money is definitely collecting Steel Platebodies in the wildy. However, hunting imps is pretty good too once high hunter
Best way to make money on an iron man is being efficient, don't just go and collect steel platebodies for hours and hours, start planning out your goals.
Steel platebodies for starting cash for runes. Then train magic at lava dragons for prayer XP also. Magic is essential so knocking it out of the way opens many money making methods. Good luck man