They will need both. Looking at the chart above for the Legendary, to craft 1 copy of the card you need 3 things:

a) 50k Aura
b) 40k Iron
c) 37,500 Cinder

And on the common card, if you want a level 4, then you need 30 copies and your math is correct on both!

If you have any more questions, then let me know @hobotortoise and thrilled you are looking forward to the release!