Personally I just check the marketboard for premelded host weapons, and for each of the 4 relics I've obtained, the melded weapons have been roughly 120k. Checking the prices without, and also the cost for materia, it quickly became apparent (at least on my server) that I was losing nothing by buying this way and forgoing finding a craftsman to help me out. If you happen to have the requisite materia and/or weapon, obviously your situation maybe different, but I suggest trying the simplest route first.