High Levels Alchemy
Runes: 4 fires 1 Nature
Exp Per cast: 65
Est Casts: 197,602
Est Costs:
59,280,600 For Nature Runes at 300 ea
Fires: You can wear Firestaff
Yew longs: 108,681,100 At 550 Each


Now, Starting with 20 mill you could get..

Around 18,181 Yew longs Strung (550 ea)
And around 33,333 Natures (300 ea)

That's with spending 10Mill on each of them.