Normally it costs 1688 to smelt one bar using materials bought off the Grand Exchange. You can make -306 coins each at current market price, 420 coins if using the Blast Furnace, -591 coins if using Superheat Item, assuming a Staff of fire or equivalent is equipped.
Members may opt for smelting Mithril bars at the KeldagrimBlast Furnace rather than a normal furnace or Superheat Item due to the Blast Furnace only needing 2 pieces of coal per mithril ore. Though this option means more mithril bars can be smelted per inventory, it is much further from the bank than some normal furnaces in Al Kharid, Falador and Edgeville. This can be counteracted by using the bank in the Grand Exchange, and requires a usage fee if a player's smithing level is under 60. It is possible to smith up to 18 mithril bars in one trip using the Blast Furnace, coupled with a full coal bag, 9 coal in your inventory and 18 mithril ores. (27 (in bag) + 9 = 36 coal + 18 mithril ore = 54 spaces in total) This would be the absolute limit, as this is the max amount you can carry at one time while using a coal bag. However, you can bring any amount of noted coal and mithril ore along with a sufficient amount of coins, and un-note the ores for a small fee by using them on the dwarfs working around the blast furnace. This makes it possible to smelt any amount of mithril with the only restriction being the bars that go into inventory. These can be smithed into weapons and armour on the anvils, and alched. The other option is to bank the bars in Keldagrim or Grand Exchange.