The cost to create it, including risks, can be calculated...but very often players will pay more than that. Your best bet is to compare with highly-similar weapons (a +5 Emi, for example).
I prefer to compare rather than determine by quality. A +6 Homunkulus, for example, is worth a lot more than +6 Dark Screamer to any given farmer or player. The crystal value are exactly the same, however, and the Blacksmith of Mammon doesn't discriminate.