an example:
http://www.theflightdeal.com/2016/10...ing-all-taxes/
(go through Norwegian UK site instead of Norwegian USA, get charged in british pounds which makes it cheaper)
272GBP on UK site (USD $332)
versus USD $418 on USA site
unfortunately, there isn't an automated way to find fares via this method. it's trial+error, or get alerted by other people/sites