Carlos,
There are several things to consider.
First, Nias is often faked because it is a unique and exotic culture, weapons, jewelry, statues.
Second, good quality, but not truly authentic weapons are mostly going to be mid to late 20th c and will cost in the $1,000+ USD range. Authentic pieces from the headhunting days will be MUCH higher and are very, very rare.
If you want to learn, spend some time on this forum. Use the "search" feature. Get to know some good dealers. Buy some books. Here is a good one on Nias.
http://www.ebay.com.my/viItem?ItemId=290092374953
Check around, you might find it cheaper.