Shouldnt cost more than maybe $10k, and theoretically less.
AROS was ported to x86-64 by one person in a few months for only a couple of thousand. The same person also ported it to ppc, along with drivers, again in a few months for about $4k (iirc).
People seem to vastly over estimate the cost of these things. Porting to another architecture is a different kettle of fish than developing from scratch. The majority of the work is already done.
