Well i used fire strike on my pure for 60 : B U L L E Ty
And that took me about 15k casts or so.
Which is only 450k for 60 magic and 48 hits.
So 450k for 60 magic isn't bad at all.
But you want 55. So that should be around 8.5K casts which is 425k for 55 magic and around 45 hits.
So that's rather cheap.
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Or you can do as Ryan posted and craft the runes yourself creating an even cheaper way.
OR, you can kill elemental wizards south of fally collecting runes. Even cheaper alternative.
Or just spam someone for runes

lol.
I have all the runes you need for 55 just for metal drags and barrows

lol.
Goodluck anyway.
Hope i helped.