It still costs the same. You can substitute the +1 bonus with an equivalent +1 bonus such as flaming, frost, corrosive or guided. So if you had a flaming amulet, I guess your unarmed attacks would also do 1d6 flaming damage.
I know the rulebook says a Amulet of mighty fists can be built without the normal +1 but I cant figure out how to cost the item without it - I am obviously missing something - can someone point out to me how I work it out
regards
Cee