1. Vendor Price

If you can sell an item to a vendor it is automatically worth at least this much. Most items vendor for a ridiculously low price so the floor is set low with this metric.

2. Cost to obtain

With gathered mats this is a cost of repairs to gear. One can argue that one's time should have a cost associated with it but everyone values their time differently. Even mind numbing activities like farming shards only incur a base price of the damage/cost to repair your gear once one is done farming. Any additional cost is just that - something appended onto the price artificially.

3. Cost to list

When using the market board one needs to recoup at least as much as they incur in items 1 and 2. Current MB taxation is listed at 5%.