How would you calculate the market price of a magic item when only the crafting price is given? Some books like Magic Item Compendium give market prices, I know, but other books still have magic items not represented in that book, e.g. the Crown of White Ravens (and other related martial items) from Tome of Battle. Is there a standard way of making a purchase price for it as opposed to a crafting price, or is it more or less DM's call?
The crafting rules have you spend 1/2 base price in materials and 1/25 base price in XP. Thus you can determine the base price from the crafting prices by multiplying the crafting material cost by 2 or the XP cost by 25.
Most of the time, the base price is the market price. Only in very rare cases would I recommend fiddling with that too much, unless you’re very careful.
Magic supplies for items are always half of the base price in gp and 1/25 of the base price in XP. For many items, the market price equals the base price.