I calculated how long it would take to get from 51-91 Runecrafting on my guy (cuz that's what lvl I was at), how much money you would spend, and how much profit you would make.
On Runecrafter Pro (rsbots.net)
51-82: Graahk Runecrafting
At 17k xp per hour (until level 75, then it gets higher), 135.2 hours (2.3 mil xp)
255,410 Nature Runes (not counting extra runes)
Essence cost (at 153 gp per ess): 39,077,730 gp
Nature cost (at 241 gp per nature): 61,553,810 gp
Profit: 22,476,080 gp
82-91: Double Astrals
At 20k xp per hour, 174 hours (3.5mil exp)
400,201 Astrals Runes (not counting extra runes)
Essence Cost (at 153 gp per ess): 61,230,753 gp
Astral Cost (at 158 gp per astral): 126,463,510 gp
Profit: 65,232,757 gp
So, in retrospect, it will take 309.3 hours and you will make 87,708,837 gp.