Reforging cost is equal to the sell price of an item to a vendor.
How do I determine the reforge price for an item that cannot be sold to a vendor (for example, items bought with tokens)?
It costs a flat 10g for unsellable items that don't have a sell price listed on Wowhead and can not be sold to a vendor.
Every item has a Vendor sell price, even if the item is ineligible to be sold. You can look them up at WoWhead.
If you'd like to check in game, simply walk up to a Reforging and queue up a reforge without finalizing it. It will tell you the cost.