came across this as i was searching for something else, the link shows a general cost of all lengendaries as of the moment, then another link on that page brings you to a website with the actualy calculator which allows you to input component prices
seems pretty useful for gauging the cost :O
http://guildwars2.mmmos.com/index.php?page=view&id=4870&title=total-cost-of-each-legendary